Reno FR makes this fight extremely easy since the bosses can quickly rack up his force time bonus. Just get an off turn damage dealer or tank with counters and you're good to go
I went Sherlotta as a third mainly because I don't want to slowly build the FR gauge
I removed the following passives
Kain- Buff Speed Up, Break Speed Up, Heaven's Trotting Knight
Reno- Buff Speed Up
Sherlotta- Buff boost speed all, and I think she has a buff speed up passive as well.
Essentially just unequip any passive that has the word Speed in it.
0:00 Setup
Use Kain and Lotta first turns to build FR meter and go into BT+
Use their second turn to use call abilities and have Kain jump into the air
Do NOT use S1 with Reno
3:37 Reno FR time
Kain jumps into air again on his turns
Lotta uses LD and Chuck Staff
Reno Spams Pyramid on his turn
6:52 Summon Mode
Pop summon when the FR turns are low and the bosses can't get another round of attacks in. Alexander summon will also heal any residual damage you took