To start with, you absolutely need all of the relevant categories specific upgrades, as well as those for relevant stats.
The strats I used were rushdown tactics where I didn't give em the the chance to call for backup. Here are the ways I've done this:
Omnicellular: just wait till assimilate is ready, if you've upgraded properly, you should have like 2.2k vit, which makes it 1 shot, even if you're missing a couple upgrades, you could additionally prime before using it to really ensure 1-shot.
Multicellular: Have your loadout be all the fortitude-scaling DoTs. Apply a couple relatively minor DoTs, but hold off on unleashing more until you have everything ready at once and they have just started charging a precise shot. Than use specialist strike and every poison DoT at once, they should die before they call for backup
Complex: loadout of the dex-scaling moves, prime-up and specialist strike. Hit em with one of the cillariry moves (don't recall which) making them around 24k hp. Than wait for you moves to charge and for them to start an attack, hit em with specialist strike, prime than fast attack, than spam your combat moves and they should die before they call for backup.
Human doesn't fight them.
None of these took me more than 4 attempts at the fight to figure out.