How am I supposed to defeat three codexes and three archons, and three heavy robots without using counted ammo like grenades and not being hit? I'll need grenades and medkits it the last fight, and this is not the last fight. I can put one archon to stasis, dominate second archon, try to hack one of the heavy robots (~70%), set both of my heavies to overwatch. But obviously, it is not enough.
Update: i'm asking this question because of final mission. Enemies will respawn in the final fight, so i must conserve ammo and medkits, otherwise i will be overwhelmed by packs of codices, cryssalids, mutons, those big daddy suits etc etc.
Best Answer
If you're intent on conserving consumables for the final fight, you need to get maximum use out of your other abilities. There's no timer here so, unless you have something Hacked or Mind Controlled, you can wait for 5 turns to let everything recharge.
If you know where the enemies are, take your time and position your squad before the fight. You want everyone in the right places (ideally spread out as all your enemies have devastating aoe attacks) so when your first move triggers the pod, the rest of your squad start killing.
Better still, locate the pod with a stealthed ranger or a battle scanner.
Don't put your grenadiers on overwatch. Ever- they're awful at it. If you want to avoid damage, they need to be proactive and start taking down enemies. Ideally they should use a cooldown-based cannon ability; you haven't mentioned their builds but opening with chain-shot, rupture, or saturation-fire will get you kills and deal with the mecs armour.
Snipers should either open with serial and should finish everything that ends up low on health or should use killing zone. If they are a gunslinger, use every ability they have to dish out all the damage you can. I can't stress enough that the fewer enemies there are at the end of the first round, the less damage you will take.
Specialists are best off opening with support protocol followed by hacking a MEC. It'll draw enemy fire and this way it can't shoot you.
The Avatar itself is monstrously powerful. It also has potentially limitless regeneration so while it's not indestructible, you can place it near the frontline (behind cover of course). You should also consider using rift instead of mind control- it can destroy multiple targets.
Finally, find something better for your psion to do than Stasis. Try to get kills with something destructive or use inspire to give the avatar another action.
If you still can't win, you may need to re-evaluate your stance on using consumables. You're not supposed to be able to clear difficult fights without them and a single flashbang or EMP would make this fight significantly easier.