My two flyrants also get killed most times but the still get the job done even as a very expensive bullet magnet. Their to fragile to just fly towards an enemy army, but imagine this:
You deploy your 2 tervigons(maybe outflank one if you can afford it), 2 flyrants and 3 mawlocs. Tervigons get their endurance or iron arm on, flyrants fly of the table and mawloc's burrow. Next turn you got 2 flyrants and 3 mawloc's coming unto the table and also a doom (pod) and 2 groups of ymgarl genestealer on a 2+ (Hive commander). I won a lot of games with first blood, a tervigon on my objective and a few mawlocs contesting and getting linebreaker. Who cares that the hive tyrants are dead!
But tyranids cannot afford a lot of mistakes...
1: You need to pick the right psychic powers (sum times its better to try to teleport a tervigon instead of making him tough, you need to get in their face!)
2: Deployment makes or breaks an tyranid army.
3: Reserves: First let the mawlocs do their thing and let them move units towards each other (doom) or wreck transport and use a "run" move to get them in cover. The can also clear a piece of terrain to make sure that those ymgarl genestealers can be placed. Always let the hive tyrants fly over terrain. If the drop the can still claim a 5+ coversave if not more.
4: Go for the mission!
And then you still got to play it smart:
- Against daemon flying circus I once made a big blob of my army with an extra amount of spawned guants to make sure that those flying
MC's and helldrakes could not vector strike because their was no way to place the model behind my army.
- I once used "warp speed" (biomancy) to add a d3 Initiative to make sure that my tervigon survived a Jaws of the world wulf.
- I use "smash" with my mawloc's if i dont want the enemy unit to lose combat. Next turn I use all of my attacks and then use 'hit and run' if their still locked.
- Once made a conca line with ymgarl genestealers and assaulted true cover. First ymgarl genestealers got killed but the enemy needed to pile in towards the ymgarl in the back and got so close that my termaguants could assault next turn.
- faced a wave serpent list and gave him first turn with nightfight. I deployed waaaay back in a corner with 2 flying hives and 3 mawlocs and the two guant units behind a ruin(tervigons in reserve). With nightfight I got a 2+/3+ coversave and only a few wave serpents could shoot at them. My first turn I flew of the table and let those mawlocs burrow. I only got a few guants on the table and his second turn he could shoot at nothing!
Hope it helps!