Imperial Guard - Advantages
Shooting. It's that simple. Whilst Imperial Guard don't have as many AP3/2 guns being thrown at Tyranids such as Missile Launchers and LasPlas Razorbacks, no matter what type of army you are fielding, you're going to have a lot of S6 firepower from the Chimeras. The plethora of S7 firepower readily available to the army from Autocannons and Hydras is also going to happily put wounds on T6 creatures. Sure they get their saves and FNP but when you force enough and nearly every unit in the army can do this, well things fall over dead. Back this up with orders to overcome the BS3 of Imperial Guard and those wounds will start to fall.
That being said, there is a limit to how much S6/7 guns can do to T6 models getting their normal saves and FNP. So lets add in the plethora of meltagun options and lascannons the list can access (hi Vendettas!) and tough targets, well they aren't so tough against Imperial Guard and remember, this is what Tyranids are paying for. The meltaguns are obviously a later game option, you don't want to move too close to the army that's better in combat after all but having the option of denying saves to things like T-Fexes regularly forces Tyranids to work harder for their cover.
What's great about all this firepower is it is mobile and has significant range. This means Tyranids are going to find it even harder to get across the table as the firepower that's hurting them is moving away from them and always moving to try and get clear shots (so no cover). Furthermore, unlike Space Wolves, Imperial Guard regularly have ranged anti-infantry options as well as the closer ranged options such as Heavy Flamers. The combination of these two makes the little gribblies of Tyranids far less effective and scary and leaves the bigger guns happy in the knowledge that even if they don't down the MCs quickly, there are lots of options to deal with the smaller guys. Blasts in particular are pretty useless against MCs. You've got a S8 AP3 ordnance blast? Amazing, you have less than a 20% chance of inflicting a wound on any given T6 creature but you could be killing lots of little guys quickly.
The heavy flamers on tanks are also really important as it allows for mop-up duty. Tyranids are going to break through the lines at some point and being able to tank shock them into nice template formations (or even get them right after they've assaulted a tank) is going to kill a lot of them quickly. The Quad flamers from Platoon Commander squad also fit this bill nicely. Whilst Imperial Guard do have excellent anti-infantry at range, it's also very effective against the big bugs (medium/high strength and good rate of fire) so can often find itself overwhelmed with both the big bugs and gribblies. The mass amount of templates EVERY Imperial Guard list fields overcomes this nicely.
This leads right into the major advantage any army has against Tyranids - good mech. I'm not a fan of IG foot lists and think they will struggle a lot against well-built Tyranids but Hybrid or mech Imperial Guard will give Tyranids fits. Not only do Tyranids have few effective anti-tank units but there are so many separate tanks on the Imperial Guard side of the field, they will rarely be able to deal with them all. This means the Imperial Guard army, like the Space Wolves before it, will have much more options for dictating play. Normally the side AV of their Chimeras can really limit their mobility. No one wants to expose AV10 if they don't have to but with the limited mobility and breadth of anti-tank firepower the Tyranids bring to bear, this is less of an issue.
Tyranids - Advantages
So what do Tyranids have going for them? This and that and they will look pretty similar to what they can do against Space Wolves. The main one is of course close combat and this is a much greater divide than seen between Space Wolves and Tyranids. For one, cover is less of an issue. Sure striking last sucks but against Guardsmen? Far less of an issue. Obviously Straken with his counter-attack bubble can change this but even then, getting hit by WS3/S3 attacks is a lot worse than lots of WS4/S4 attacks on T4/3+ save platforms. Even the mighty Tervigon and T-Fex are going to be pretty safe in combat against Guardsmen as hidden fists are not a possibility in most squads. Whilst this is a great asset it is also an annoyance. Any units which you drop a lot of points on such as Warriors, Raveners, Tyrant + Guard, Genestealers, etc. are very likely overkilling your opponent which leaves these units to get shot. Multi-assaults are of course a great idea to help stop this but without tanks this is relying upon you killing tanks and your opponents making mistakes.
Combat goes even further and helps Tyranids deal with tanks. We all know assaulting tanks isn't the most reliable way to deal with them but when they are so many tanks on the table, you have to do something. This technique is especially good if you can engage a tank parking lot (particularly if it hasn't moved) with units with Rending (Raveners, Genestealers) or the ability to punch through tanks regardless (Tyrant + Guard). The problem here of course is you leave yourself in template me formations but you can do a lot of damage in one hit.
Imperial Guard have no psychic defense (once new SoB hit at least) so powers like Catalyst, Onslaught and Paroxysm should be going off regularly. Paroxysm isn't going to be as beneficial in close combat as it usually is but being able to effectively shut down a unit's shooting is nice. Onslaught can really help the overcome the lack of mobility on the Tyranid army and get Hive Guard/T-Fexes into side arcs of Chimeras. If you can start forcing side shots, you'll go a long way to hindering Imperial Guard in midfield. Catalyst is of course the kicker and gold star for Tyranid psychic powers and being able to cast it without any defenses to stop it is great. If one can hide their Hive Guard behind MCs or terrain and cast Catalyst on the Tervigons/Genestealers, the Tyranid army has a much greater chance of operating effectively. The problem again becomes the Vendettas and making sure they can't scoot around and either bring MCs down with concentrated firepower or destroy the hiding Hive Guard.
Heavy Venom Cannons are pretty decent against Guard with their bigger boxes and massive aeroplanes. Whilst the Harpy is a lacking choice in its current rendition, twin-linked HVC will do decent suppression against Imperial Guard. That little blast has an extra inch of wiggle room on Chimeras and much more on Vendettas/Valks. Add in it's one of the few units which can aggressively get into the side arcs of Chimeras and you can really hinder their mobility. They of course have survivability issues but if your opponent is shooting them, your Hive Guard, Tervigons and T-Fexes are all pretty safe.
Then of course we get to the Zoans, Hive Guard and T-Fexes. These guys are simply overwhelmed against Imperial Guard, there are just too many tanks for them to take down by themselves. As always however, they are great at reliably putting out damage on five different targets a turn. It's just a matter of keeping them alive. Aeroplanes are the prime targets for T-Fexes. You'll be able to get in range of them easily and have the strength to bring them down. Hive Guard/Zoans need to deal with everything else at range backed up by any other suppression fire you may or may not have. This generally means dropping Chimeras upon which your little guys can feed. Whilst Hydras and backfield Artillery would be good targets as well to reduce the incoming firepower, Hive Guard will rarely be able to actively access those units and as such, are better left to T-Fexes if they down the aeroplanes. Against IG, you need to kill as many tanks as possible. As discussed before, they can really disrupt your movement forward even though you are fearless and ruin all your plans in objective games. The backfield tanks are less likely to do this, they are in the backfield afterall so it's all about dropping those Chimeras. The more you drop, the more you expose of your opponent's infantry, the more firepower you decrease coming towards you and the more you restrict your opponent's mobility. Sounds simple!
Using these units is going to be key to victory for Tyranids. The longer they survive, the better. Hiding Hive Guard is of course paramount and ensuring T-Fexes have cover from Ap2 shots is important as well. T-Fexes will stick around for a long time. A single Vendetta needs at least two whole turns to kill it and that's assuming you get six hits, wounds and failed cover saves (average is 1 wound a turn). By using the tactic of cornering + onslaught, you may still me pretty static in terms of getting side shots, but you will generate side shots if the Chimeras push into midfield. If by doing this you stop them from moving into midfield, well win for you as you get to move up and potentially protect objectives. If they do, side shots galore! They can still of course refuse flank you which is where tactical ability comes into but all is not lost with Tyranids against Imperial Guard - as long as your T-Fexes and Hive Guard stay alive long enough.
Oh and a unit which gets talked about a lot lately. Biovores. They love Imperial Guard as they can hide pretty easily and launch firepower which wrecks clumped Guardsmen very nicely. Much handier against Imperial Guard than Space Wolves where you can generate a lot of wounds on a single squad due to the nature of the squads themselves (i.e. not small unlike Long Fangs). This otherwise can overcome the Tyranid deficiency in ranged anti-infantry though when Guardsmen pile out of tanks they are usually dead anyway. Tyranids munch crappy infantry from combat in breakfast after all.
Conclusion
It is again a match up determined both by who can apply their strengths better (no duh) but also a game of dice. If the Tyranids have good dice early (or the IG bad) they can advance across the board relatively in-tact and we have a game on our hands whilst if the dice are poor, the Tyranids can be stalled as early as Turn 2. This really hinges on a few things. Whilst Imperial Guard firepower is generally quite good at forcing saves on the T6 models of the Tyranids, they are still regularly getting their armor saves and FNP. If there are only a few AP3/2 weapons around, such as Vendettas, the Tyranids can attempt to remove suppress/remove them early and rely on their saves and FNP to keep them alive. Being able to hide Hive Guard is also huge for Tyranids as it keeps some of their best anti-tank hidden. Imperial Guard do have some mobility to access these guys but if you place them properly, to do so they will need to expose their side armor more often than not.
This match up can often feel like a game of dice but with proper target priority for Tyranids, you can play to the toughness and durability of the army and force the Imperial player to mobilise their army rather than sit and shoot. For Imperial Guard it's all about dropping the Hive Guard ASAP. The sooner you decrease the Tyranid's ability to shoot you out of your boxes, the better. Use any mobility options such as outflanking, reserving planes, etc. you can to make sure you can access Hive Guard where ever they hide. You'll have the tanks to soak up a few kills but if they last an extensive period of time, you'll be hard pressed to stop them and the horde of gribblies + MCs coming at you.
In the end Imperial Guard has the advantage but it's not as great as I believe everyone perceives it to be. You can certainly make some lists which just chew through the common Tyranid builds with masses of AP2 firepower but you'll often find you are pretty static and suffer against other armies. Tyranids on the other hand can deal with Imperial Guard decently well if their anti-tank survives the first few turns and it does have the durability to do so. Once you start knocking out the planes and midfield tanks, Tyranid combat superiority can come into play but you are still on the back foot in regards to dealing with all the tanks and late game tank shocks.
