The first boss is hard at first, but once you figure out the pattern, it's pretty easy to finish him off. During the first part of the battle, he won't do anything, other than absorbing your attacks, but after bringing down his hp, then he'll start to attack. The best thing to do (imo), is to keep dashing, and only attack at certain times. As for pins, you should have some long range attacks, such as Meteor Magnet, Thunderbolt, Force Rounds, etc.; close range works too, but it's sort of a risk, as the first's boss attacks are pretty much close range as well. You should also think of brining a cure drink pin, in case it gets risky.
Hope this helps.