You have two options:
Use the Crit stones from the NPC in each main city, I used Fortified incinvible resonance stones.
Save up lots of Hurricane powers and buy the Ultimate Invincible stones from the Scorching Wind Desert altar.
In 30/40 arena, I honestly wouldn't worry too much about it.^
Warriors are fun to play around with: Shackles, attack, Stun, shackle.
*21 set*/Sleep staff/Cast Reduction Staff might be of use.