Put some points into Backstab, so the cooldown is about 4 seconds, then you should have around 1.5s uptime.
Backstab and put down broadside, rinse and repeat every 4th second.
Backstab HEAVILY increases the damage, around 15k magic damage.
You can melt bosses very fast with it, imo on of the fastest boss killer in the game, and one of the fastest farmer (and doesnt feel clunky if you have Dark Pact helmet so you always have Bloodlust for hades gate).
Also, the only ways you can afford to max is Way of the Gryphon, since you wanna:
Smax Perfect Being, Broadside, Laserblade, Prismatic Cloak, Way of the Gryphon.
Enough points in Shadow Refuge to have 50% PR.
Atleast one point in Doom, I just use it for harder, tankier bosses.
If you have Blackjades Legacy, then it can be worth to max Shadow Refuge and place the leftover points in Way of the Gryphon (this needs testing). You lose some Lightning Pierce, but get more fire pierce + physical damage (and some movement speed which can be nice).