I raped last Nidhogg with Kerria+Pinkladies+Maid Cowslip.

For a Crit team, I recommend Pigmy Lily. WeakXCrit is the most broken thing in the game. Crit deals x1.5 damages, weak deals x1.5 damages and WeakXCrit deals x2.25 damages. Without any buff.
With buff, I guess you can just do that calc: (1.5 + (all crit buff) / 100 ) * 1.5.
First 1.5 is base crit damage.
(all crit buff) / 100 is the girls crit damage buff in %, divided by 100 to get a decimal value.
Second 1.5 is weakness damages.

In another words, weakness increase your Crit damages by 50%. So, it's like +one half of your whole Crit buff.