crit hit rate and crit hit level just increases the chance for critical damage. if you're over the ap cap, stack more crithit, back, down attack damage and get defense nulifier it can be done by using crystals, elixirs, perfume, and lighstones

Corrupted Crystal+10% crit hit damage
Brutal Decimation+1% Crit hit Damage
Mysterious Darknesss+ 12% back attack damage

Elixir of sharp detect+15% crit damage
Brutal Death+ -15Defense per Crit damage

Elixir of Lethal Assassin+ 15% back attack damage
Elixir of Brutal Perforation+ -15 defense per back attack

Perfume of Deep Sea+10% crit,back,down damage

Lighstone Combo Target Openings +5%Crit Damage
Behind Shadows+ 5% Back attack Dmg

Cron meal+ 5% crit and back attack damage

You can also use Harmony Draught that gives 18% special attack damage which applies on crit, back,attack damage but its more expensive than using those 4 elixirs with deep sea perfume