What are some of the best PAs to have equipped for boss fights and big damage?
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You don't use launchers for bosses unless you're able to hit two bosses at once with Cosmo Breaker.
For rifle you use Satellite Aim or Positron Blast depending on the fight. You can use Impact Slider during movement phases to keep dps going while giving yourself iframes.
Photonic laser on launcher is good for anga when the boss swaps to rifle resist or you can hit a bunch of mobs like on falz elder hands. As stated above though the rifle is the weapon for bossing. Launcher is for regular mobs and most other situations. Satellite aim is a gunner pa, satellite cannon is what they are talking about.
People don't give Steady Shot enough credit. Yes it takes a while to fully execute, but so does Positron Blast, the latter of which can miss some faster moving bosses. Steady Shot on the other hand can easily nail bosses who dodge and jump around a lot, and it allows you to move while using it so you can dodge yourself. As long as Standing Snipe was active at the start of the PA, the damage bonus applies.
My advise, use Positron Blast when you know you can use it and line up the shot so it hits multiple times. If you're unable to do that due to the boss size, position, or speed, use Steady Shot.