Might of the Guild.

Zsymon

Does the 20% damage bonus from Might of the Guild become active after casting Magelight? If so you could basically just retoggle Magelight before every spell?

McDonuts

Yes it does. However, the damage boost is calculated weird. I only have 10% right now, but adds way less damage than the 10% the Resto staff passive gives. That one doesn't truly give 10% either, so I'm thinking it boosts spell power or something by 10%, or adds 10% of the base skill damage.

MAOofDC

The skill reads
MIGHT OF THE GUILD II
WHILE USING MAGES GUILD ABILITIES
Casting a Mages Guild ability increases Spell Damage by 20% with the next attack. Effect lasts up to 8 seconds after casting a Mages Guild ability.

Looks like that would did it.