Just a few of the bosses in PSO2 as a Gu/Ra!
I focus Weak Bullet and Chain on enemy weak spots while using their elemental weakness attributed on my guns to get the most damage multipliers.
As for my skill tree, basically I just maxed out Weak Hit Advance 1&2 from the Ranger tree.
In the Gunner tree I maxed S-Roll Combo for the 200% JA bonus,
Grab the gear for bonus damage.
Maxed Chain Finish for more damage when I end a chain.
After that I focused Perfect Keeper before Zero Range and just focused on not getting hit more than going up to them.
Skill tree: bit.ly/1uhPz8c