best user for Berkut lance+DC?

It's probably best to wait and see what his stats are before deciding.

His A gives him 7 defense and resistance at 80%. Why would you even?

I don't think many are willing to give him Berkut's Lance and DC when he already has a weapon with DC...

Best user for Berkuts Lance is Berkut, duh.

Berkut for sure, especially if you've got the DC to turn him into a capable tank. Give him the Def refinement and Quick Riposte and he's set to destroy most reds and blues without too much trouble.