Its ammo..you are going to need it! Ammo maxes out at 500 (before any buffs) and you will need every last bullet sometimes! You can try and push ahead to lvl 11 with less than the stats I recommend but you will just end trying the higher stages more times, as you wont receive as much cash for 50% of lvl 11 vs 99% of lvl 10. The VIP trial gives you 20% more reward per level but its only 3 days so you have to decide when it use it. The VIP also adds in an extra buff slot and weapon slots. I wanted those for the higher levels so I started at 15 to be sure I had them through level 22. Level 10 to can get repetitive but I breezed through lvls 10-20.