Summon damage isn't halved against multiple targets like Vivi's spells are, but your summons will do 1/3 less damage for half-animation. The Boost ability will make all summons full-animation and full-damage....and I understand that summons do less damage than regular attacks because summons are directed to the entire enemy party
Also, as mention previously, get more Gems. They make the biggest difference. If you really want Dagger's summons to do more damage you should grab as many gems as possible.
