1. every bosses are different....example light raid hard to build up rage and rage mode longer while others are shorter...so no definite answer for this
2. when u can't nuke the boss down in few rounds and bosses are not immune to status effect
3. they are the life saver, 1 or 2 in team will do, while i personally prefer running full offensive team~
For burst yes but when u get unlucky.....bosses triple shot ur soul.... then it will be dead = 0 dmg deal~
P.S. I'm the one using Solomon for sucking dick... ability dmg boost with rage skill(overdrive from Siegfried) ain't joke~
my dark amon and solomon each dealing 90-100k per rage skill~