Hi guys!
1. Make sure you are in a group with your arenamates
2. Go to a hero doll (DPS-testing dummy)
3. Queue for an arena match
4. Shoot the doll until Brain Freeze and Fingers of Frost proc and try to keep them active until you get an arena invite
5. join the arena
6. Now the procs are still active but without a time limit. Your first attack will therefore benefit from the procs.
I only tried it with Mage, so i don't know if it works with other classes/procs, but i think you should try it!