First of all try to get to level 5 in endless mode without upgrades. just dont buy them.
once you do that you will have 5 upgrade points to spend,try some combos - bow kick and clone,hammer jetpack clone and sword clone.
once you have tried them all you will find one style that fits you the best. then just use that style.
1.to kill a hammer bot attack him after they swing,if you missed run back fast and try again.
2.allways aim for the head or the torso.
3.as you will have some hours in to the game you will get better,so dont give up fast