Farm dungeons for Artifacts. Upgrade them and equip them onto your champions.

Farm potions and Ascend your champions. 

Oh and, throw some more money at Plarium while you are at it. If you buy million sacred shards, you are bound to get at least something usable.

Without knowing who you have, it's hard to give you much feedback.

Arena is largely divided into three types of teams - ATK/SPD, DEF, and DEF/RES. ATK/SPD beat DEF, DEF beats DEF/RES, and DEF/RES beats ATK/SPD.

ATK/SPD teams, at higher levels, require speeds of around 300+ to beat other ATK/SPD teams, and generally make short work of DEF teams - although they usually get crushed by DEF/RES, since they really need the DEF down debuff in order to cut through Krisk/etc... My version of this team is Arbiter, Nethril, Big'un, and Cruetraxa.

Pure DEF teams work very well against other DEF/RES teams, since they aren't devoting stats to resist - my equivalent of this team is Sepulcher Sentinel, Rhazin, Ignatius, and Brago.

I don't have a DEF/RES team yet, but I plan on converting my above lineup, once I get the right gear/champs. SS/Rhazin are being used on my CB team, and Ignatius for NM solo farming, so they can't really waste stat slots on RES. Going to switch the team to something involving Miscreated Monster and Brago, though I haven't fully fleshed it out yet.

Idiom: You become what you fight.