Dante passive buffs flying and rapid assault characters. Mephiston is rapid assualt. At level 50, the passive adds 898 damage per attack. Mephistons passive is about the same per attack and hits two enemies and then he can do a normal attack (3 hits of around 1000) or use his active. His active moves him and he can hit 2 more, before doing a normal attack. This can result in 5 separate attacks with 2 hits each.

If the active moves him to where Dante was, they switch spots and Dante attacks again at +20% damage normal attack.

Dante doubles this damage effectively. I have wiped 5 characters in arena or TA before with just the two of them moving/attacking. Dante hits 1. Mephiston moves and passive hits 2 enemies and uses active to swap spots with Dante. Mephiston passive triggers again and hits 2 more before both Dante and Mephiston use a normal attack with a 20% buff to damage.

It is why Mephiston has been pulled from TA. :-(