Create monsters that have attacks that break shields. Done. Kaede doesn't have that extreme of an HP, so you could do that without denying too many other teams.
A 'buffs disabled' could work as well, though overkill. If they wanted to really just target the new Dragoncallers, they could just have an attack that breaks any shield over 50%
A mix of heart eaters, resolve, awoken skill disabled and other nasty currently existing debuffs could really screw her up as well.
That's Pandora's Box right there.
You are essentially describing one of the two possibilities:
1) The shield is a buff, that can be dispelled. Problem here, is that their shield is part of a leader skill and not a buff (it doesn't show in the buff window). I'm not sure that the coding exists exists to dispel a leader skill directly (there never was a need to do that).
2) You are advocating that monsters do true damage to ignore defensive shields. The coding is surely there, as players have always been able to do this. However, this sets a very bad precedent. The moment that enemy true damage exists anywhere, the entire meta of the game has to shift permanently to "As much HP as possible, with highest potential attack running HP boosting leaders". The game has always skirted around this issue by simply having monsters do silly amounts of damage (aka 300k+ damage killshots).
Also, breaking shields over 50% would also claim Indra and Kush as innocent victims here.
-- Comboing of existing mechanics:
Awoken disable is meaningless. All of their power is derived from the leader skill. Full activation can be achieved in an awokenless state.
Debuffs: The problem here is that GungHo messed up bad and made an niche, but severely OP skill finally available on a REM card that no one cares about. That card is 2271, Maeda Keiji. Inherit Orb Refresh onto any card on the team to get out of any debuff situation. 99% gravity --> LKali board change --> lock all orbs? Orb refresh your way out that every time.
Resolve: Reasonable, but an annoyance more than anything. I could see more bosses like Seraphis in the future, where players have to chip the enemy's health into a narrow zone for a kill shot. However, there is nothing that really stops Dragoncaller teams from keeping the 75% up constantly and slowly grinding their way to the target zone.
I really don't envy the situation GungHo has placed themselves into here.