Warlord or Giant Slayer

Giant Slayer mastery description

Has a 30% chance of dealing additional damage when attacking. The amount of additional damage is equal to 7.5% of the target's MAX HP or 3% of the Boss's MAX HP. The additional damage can be dealt with each hit, and it is not counted as a separate hit.
Damage based on [enemy MAX HP]

Description of the Warmaster mastery

Has a 60% chance to deal additional damage when attacking. The additional damage equals 10% of the normal target's MAX HP or 4% of a Boss's MAX HP. The additional damage can only be dealt once per skill, and it does not count as a separate hit.
Damage based on [enemy MAX HP]

Masteries against Bosses

These are the two best masteries, each increases damage by at least 1,000,000 damage over 20 turns.

Which to choose? If A1 is 1-2 hits - take WM. If 3-4 hits with A1 - take Giant Slayer.

For example, Kael should get Warmaster, Apothecary should get Giant Slayer.

Proc Chance Table