>>539213046
Tyranids either run down their opponent's resources through attrition or they amass such numbers that they just crush their opposition in a sudden massed assault.
A tyranid warrior would aim to have a group of escorting hormagaunts distract or drag down an isolated space marine while he inflicts a killing blow, but tyranids expect to and are supposed to suffer disproportionate casualties if their opposition has a superior position and can maintain its defensive integrity.
Basically, a single devastator should be able to completely shut down their assault for as long as he has ammunition if he forces them down a chokepoint of some kind, you're not getting passed a heavy bolter down a corridor without heavy armor. But he and his squad will be immediately surrounded and torn apart if they're caught out in the open.
Tactics and positioning are things space marines usually use to regularly defeat most of their enemies.