Ok this idea comes directly from Borderlands 2 .

In that when you head shot a Goliath, it makes him rage and attack the nearest thing next to him. If he attacks enough targets he levels up. The goliath then levels up a few times ending as a Godliath. If you then kill the Godliath outright it gives you alot more XP and better loot.

This mechanic could be applied to one type of Tyranid which in turn you could make very rare so its difficult to over farm.

What Tyranid do you suggest that will do this?

Shooting quite a number of Tyrannids only succeeds in pissing them off.

N

Not too sure but i would have to be one of the bigger nastier ones to make it work

A haruspex may work for this, in tabletop they can eat enemies to regain wounds. I don't think it'd be too far of a stretch to see one nom a gaunt or two to get health back/get more strength. Would make for an interesting random encounter that maybe needs special weapons/tactics to deal with more effectively; if it's sufficiently hard without, it could encourage more planning or varied loadouts when doing the PvE.

Plus they're terrifying with their hentai-mouths