Every attack has a chance to proc a critical hit. For a critical hit to occur, the attack roll must be ‘perfect’ (all slot rolls succeeding).
Critical damage varies based on the difficulty you are playing on. Higher difficulties have a lower critical damage output. Critical damage can be amplified through the Frozen debuff.
1-100 roll < ((Weapon crit chance + Crit modifiers) + (0.05 * Focus Points spent)*100)
In practice, A trapper using a bow with 10% crit chance and the Sanctum of Blood 10% crit chance boost spending 2 focus on an attack and achieving ‘perfect’ would have a 30% chance of performing a critical hit.
Guaranteed critical hits can also be achieved through the Hunter’s Called Shot ability.