You’ve learn to cast spells to combined with allies’ combo burst.
Prerequisite(s): Caster Level 3rd.
When you cast a damaging spell on a target that has been effected by a combo burst in the same round, you deal additional damage equal to the weapon damage used for the combo burst and the DC of the spell (if any) is increased by 2. However, you must cast a spell which spell level matches, or is higher than the multiplier of the weapon damage of the combining combo finisher. For example; if two combo finishers was used that deals double weapon damage, you must cast a spell of 2nd level spell or higher to magic burst their combo burst. If it is triple weapon damage, you must cast a spell of 3rd level or higher to magic burst. You can magic burst multiple targets provided they have been effected by a combo burst the same round and the appropriate spell is used. The window for a magic burst is 1 round after a combo burst has been completed.