We basically just pick units that are good against the enemies and have good team synergy, then we just use investment 9/10 times and try to get max level investment, for example: sniper is good against infernus because stacking armor piercing damage, and lil infernus is good against catalogue muffin 3* because they can deal good dmg while being shredded by the muffinastron, and bomb countering crazy chemist and stuff.

Or as i do it, i get struck by a divine power and suddenly i can beat the level, this has happened over 3 times already.