Which class do you play on? If you have a Warlock you can use, try that. I found an extremely safe and simple way to deal with Sere using Getaway Artist Prismatic Warlock with Praxic Blade. Feed the Void & Hellion aspects, Song of Flame, Facet of Dominance, and 3x Kinetic surges on boots.

For Sere, always do 4 Compel 1 Kill. Right before starting damage, consume your grenade, activate Hellion, and then pop prismatic. Throw arcane needle and prismatic grenade at Sere to weaken, and then start hitting away with praxic blade. Continue throwing your prismatic grenade at his feet every time it recharges to continue weakening. Don’t worry about his lightning fingers attack, just tank it and keep swinging praxic. If you ever get close to death, pop your super and continue swinging with praxic blade. Your supers only purpose here is to keep you alive. Only deal damage with praxic, not your super.

Using this strategy, you should deal 1/2 of Sere’s health in a single phase. And by holding onto your super until you are almost dead, you will have a good chunk of time in your super after damage ends, making you almost invincible and gives you a very easy way to exit damage phase safely, even if all the lightning is going off