you can beat each stage by focusing on just one minion.
most important upgrade is range, get enough and you outrange most major threats and just win cause they never hit you.
if theres something you cant/arent outranging, focus on damage so you get through stuff faster and can potentially get to the one dangerous enemy sooner.
also of course focus on unit amount. get 3 minimum, more if you are clearing the enemies but one or two lanes is left open. you honestly rarely need speed or rate, but they can occasionally bypass a hurdle. hp only bypassed a stage once for me, because the only ranged enemy did just 4 hp and i just got to as point he couldnt kill me before i wiped the board.