Cerberus is first encountered as the level boss of Mission 3 then becomes part of the boss rush in Mission 18.
Vergil trivializes this with his Yam Aerial Rave PS while Dante has to do the following:
In Mission 18 A&R will be used instead for Aerial Crosses for stun which usually happens in 5 hits. Combo A and DTE. If Ice Breath on ground happens then abuse Aerial Cross + E&I.
This fight completely differs from how it's done regularly in Normal. Both characters have to follow this:
When it comes to RG with Dante
Ensure that Cerb is left with two heads after the stun to maximize release damage, which would usually kill him. The amount of times to hit Cerb for the stun varies.
Vergil needs to use DT a bit at the start to make PS shoot faster and take out the ice-shield more quickly. Avoid attacking the paws to not make Cerb stunned early. You will lose a lot of DPS due to that.
The speed of this fight depends on your need for the Rage-meter. If you are confident with 1.5 then you can kill cerb before it could even go berserk. The knock down depends on whether Cerb being super passive.