I'm not having trouble with the enemies it's more like they take forever to kill than when I'm using either Nero or Dante. I guess I'm just using some pretty terrible attacks with him/ or not using him the "correct" way.
The thing about V is that your damage output will suck if you are just mashing.
Hit up the void and look at the damage output of Shadow/Griffin's moves. The difference in damage between just mashing Shadow combo A and doing its combo B/C is like night and day. His charge attack and skewer do great damage as well. He also has a launcher, juggle combos etc.
Griffin has his own launch move (which you can sync with Shadow to juggle) and his charge moves are solid for AOE.
Don't forget about pumping Shadow/Griffin's ex gauge as well. Gives them a nice damage boost too iirc. They rip Artemis apart when ex'd up in mission 14, Shadow in particular.
Throw Nightmare into the mix and yeah, your damage output should be solid.
With Nero and Dante the damage is focused around them individually, with V it's around syncing the damage between 3 summons to maximize output.
Man that King Cerberus fight just drags on and on on DMD...
In DMC3 you could take out the problematic heads... Does the same thing work here as well...? I want to take out the ice head...
The fire form is pretty easy to royal guard (all its melee attacks are very telegraphed) so you can use that form to build the royal release meter to do chunks of damage. You can also save up SDT for the end of the fight to speed things up.
But honestly most of the tanky bosses are in Dante's missions and imo that's probably because they are balanced around him having Faust (i.e. they are balanced around him using all his tools).
Even ignoring the big meteor attack (which you definitely can use towards the end of a fight if you want to skip the tanky phase in DMD) there's the moves like Man in the Red and Magic Hat which are active while he is fighting normally (and both activated at the same time) so act as solid damage boosters. The Man in the Red is really good for taking down Urizen's crystal in M12 iirc.