Kroot make an excellent kill team. Cheap, melee units that can also be fair @ range, with a shaper leading it all. Your looking at 17 Kroot, 1 Kroot Shaper w/ Pulse Rifle.
With Tyranids, you have to actually think. You can't just send everything in. What you do is HIDE until that Ymgarl Genestealer unit decides to jump out and kill several enemy units; then you're fine to zerg.