game gets much to easy after assassin upgrade and A.I. could be smarter by attack units barely going for the flag while trying to kill you other simple units will go for the flag more often.
Make assassin a 20-50% chance to even out difficulty.And it shouldn't kill metal hands in one hit.
To assassinate someone you need the ninja amulet from a gold chest and once you equip it you have a 10% chance of getting an insta-kill (Assassination)
A few hints to help
1:Make a balanced tank
2:If it gets to big of a firefight back out
3:Don't stop shooting until your sure all enemies are dead
4:On the final boss (The one that surrounds the bubble) focus on the upper corners first before the base at the bottom.
Rate up if this helped.