I think it's just luck, i got good stuff in farm when i picked card and when i let robot pick.
If any1 wanna try dif things to do in boat than:
-put low lvl acc as host (my friend lvl 35 won axe in sch, he was host)
-try to win as fast as you can last stage (around 70-80 rounds) or if your team not that strong wait till 110-115 round to do the final hit
-heal less, 1 aa or none in last stage
-wait 7 seconds before picking card (this usually works for me)
-try different hours to farm
-and yes i think reward sometimes depends on how much damage you do.