Mainstay’s tooltip currently reads
Deal 20% more damage to staggered enemies. Each hit against a staggered enemy adds another count of stagger. Bonus damage is increased to 60% against enemies afflicted by more than one stagger effect.
Mainstay also increases the first stagger stage to 40% and should read
Deal 40% more damage to staggered enemies. Each hit against a staggered enemy adds another count of stagger. Bonus damage is increased to 60% against enemies afflicted by more than one stagger effect.