slow bullets off + hard is for me the only way to play, love the challenge and I've "completed" the game numerous times with it, always default weapons
1 day 22 hours, research first, accuracy+reload then, money then, 1 upgrade in regen (to be sure) then 11 skill points for tree,. only buy sniper and the other 7000$ weapon.
There are a few gaps in the album right now. They will be filled in future updates.