Look at the upgrades for your dance attacks, the charged versions of them deal this type of damage and are your main method for doing so in the vast majority of cases.

There's a Mazo that speeds up the charge time for the charge attacks enough to make them actually useful. Equip it and you'll use them all the time. Your doppelganger also benefits from those abilities and deals the same damage. The doppel can also do the Punishing Strike. Actually, you'll have to be careful not to kill the giants outright with a charged Oka that way. There's also a Mazo that MASSIVELY speeds up the recharge time on Tsuba to the extent that some of them are almost always up and it stacks with another Mazo that lets them all charge at the same time.
Though if you upgrade the dance charge moves with the Mazo for that, you won't need the stance-breaking Tsuba anymore. Just Doppel, Chozusha, then charge Oka all Seethe dumb enough to come at you and yours and they'll all tip like cows.

Its a stupid challenge, use flame and earthquake, tiger maybe.
Use talisam for tsuba regen if you have them.
Otherwise its just spending about 8 nights in the mission. You cant stun them any other way.
The problem is that the finisher must kill them so you have to damage them for about 25% of their health before.
Earthquake can stun 2 giants if they are close but the finisher has a cooldown so you might miss the second finisher.

Cannons deal a lot of stagger damage but the enemies dies before the stun