My highest score is 17 (so far), and my upgrades were as below:
1. Block Arrows I
2. Block Arrows II
3. Block Arrows III
4. Jetpack
5. Energy Capacity I
6. Clone (hereafter, if I die in a round, I will temporarily halt upgrading to create another clone)
7. Energy Capacity II
8. Energy Recharge I
9. Bow
10. Arrow Width I or Energy Recharge II
Energy Recharge II (or Energy Capacity III if already taken)
I mostly use the jetpack for getting away from hordes, trying to get some wiggle room to take out Spider-trons. I find that using it for offense, especially in later rounds, will often leave you completely exposed in the middle of a horde. RIP.
So that's my strategy, anyway. I think the fatal weakness of my plan is relying too much on being able to redirect arrows. When I swing my sword to attack something, much less turn my back to an archer, I'm left wide open.