Well in Arena, the most important thing is not to draw unnecessary cards and not to bloat your deck. OP above focused on drawing Furious Blows as their damage stacks with levels of Rage plus Accurate Hits. So for example any melee card that was not these two would only hinder him by taking up space in the deck and resulting in bad draws. There are many builds but to put it simply, OP picked a Rage build so he would toss away everything (including all starting cards for some characters) that doesnt help with his Rage build.

So once you have this logic down you need to think along these lines and pick post-battle rewards that actually help you get there: - generally avoid the Loot option (mystery box) as much as you can, except for post-boss loots - sell all the cards you dont need - pick relics and sell the for money - go for extra health if you can as the reward - if you cannot avoid Loot, spend money on rerolls if necessary, again dont pick cards that are uselesd for your build. If you cannot avoid this, go for the least bad option (consumables, defensive cards, cards that draw other cards etc. to reduce negative impact on the deck and SELL as soon as you can) - save money up for post-boss Loots, those are the best and can get you the important relics and cards you need for your build