It can be difficult to kill him when he's alone and your entire team has pain, because he gains so much defense and tenacity. Characters that gain or give tenacity up are helpful, because once they have tenacity up, they pretty much can't gain Pain.
Also, if you have someone in your group that can cleanse Pain (Revan, Ackbar, Talia, Chirrut, Rex, etc) that will help you deal a lot more damage to him. If you're down to just Sion, I recommend letting him use Held by Hatred (resetting the CD to 15), and then try to nuke him asap, especially with tenacity down. You should be able to kill him before he gains 15 stacks, he will revive and attack regularly, and then hopefully the next round of attacks can kill him.
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Also, if you have someone in your group that can cleanse Pain (Revan, Ackbar, Talia, Chirrut, Rex, etc) that will help you deal a lot more damage to him. If you're down to just Sion, I recommend letting him use Held by Hatred (resetting the CD to 15), and then try to nuke him asap, especially with tenacity down. You should be able to kill him before he gains 15 stacks, he will revive and attack regularly, and then hopefully the next round of attacks can kill him.