Shields are the wrong strategy. They don't work against bomb or Dreadbomb damage and can only prevent damage from the hit the boss does himself. Reflect damage doesn't do anything as well. Bomb damage can't be reflected, only the normal hit damage from the boss. Grappler is pretty useless in this fight, first candidate to be removed from this team.
The bombs can't be removed or blocked. And for increased def the same is true as for shields, it doesn't work against bombs. So the A3 skill of Marked has little benefit. Her A2 indeed has, as the bombs can be resisted and decrease accuracy helps to do that.
I suggest to use the following champs:
Elhain. Not really good at this fight, but lvl 60, so her base stats will result in the best damage of your champs.
Ninja. Hits hard, can freeze the Dreadbombs.
Warmaiden. Decrease def and lvl 60.
Mother superior. Heal and decrease atk.
Marked. Decrease accuracy.
Be aware that the boss will remove all debuffs from himself on his first move. This skill from Bommal is on a 3 or 4 turn (I don't remember exactly) cooldown, so after he used it, you can place your debuffs. Maybe it's a good strategy to hold back some debuffs and don't use the debuffs from Marked and Warmaiden in turn 1 of the fight.
If it doesn't work right now, go and make Ninja lvl 60. He is the champ in your roster that should be maxed out next, followed by High Khatun and Tayrel. Don't invest any further in Marked and Mother Superior. They may help in the actual fight, but you will get much better champs with similar abilities while playing the game. Decrease accuracy (Marked has only the small version of this debuff) and healing are both pretty common in the game.