Take all the bosses in trials and split them into 2 groups.

1 group of bosses that is immune to magic damage/aoes. And give out physical damage that can only be countered with heavy armor/medium armor. These bosses would require melee/ranged melee damage..

2nd group of bosses immune to melee damage and require magic damage/aoes to bring down.

Also.. before entering trial run.. lock down armor/gear swapping to prevent same group of casters to change out armor.

Sure.. this will only upset ER and guilds that only run wearing light armor and aoeing everything.. but the good news is..they are the minority.. if they complain no biggie.. you need to please the majority.. SEE Politics.