Some good uses for each unit:
Gladiator: Don't use.
Archer: Don't use.
Defender: Don't use, unless you want more specialization.
Spear: Use behind defender/knight (knight preferred), good for damage from safety.
Medic: Use a few, probably one for every defender/knight, maybe a couple more.
Ninja: Use behind your front lines and in front of your snipers (helps against soldier charges + alien rain on their turf)
Mechanic: Use only in defense if you decide on cannons or walls.
Mage: Don't use, unless you think it's cool.
Sniper: Should be your main ranged unit.
Knight: Put in front, great tanks.
Just wanted to post how I used em.
Oh, and about this lvl 5 debate... I'm gonna actually help the little guy. @PoopLoops: On the bottom-center gear, you have to let go as soon as you get a little air from getting close to the gear. That'll launch you all the way to the left, every time.
"Great job guys, you restarted the drainage system." VICTORY *team is out of ammo and surrounded by hordes of aliens on all sides, only 1 squadmate is in the elevator...*
Hey, if you let Taylor pick up all the ammo, her speed can help her distribute it among all your allies. Just a little tip, not sure if it works. Try it if you want.
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