I don't need to win every battle, because I fight to win the war.
Pick your fights, and pick them carefully. Know when to walk away, because sometimes it just isn't worth it.
Only fools think they need to fight every battle, let alone win. when you are "arguing" with someone online, remember that a troll won't change his mind, so your actual audience is not even the troll, but anyone observing.
If the other people in the thread cannot be persuaded, then just walk away, and save your energy for an audience that will listen, and that can be persuaded.
Keep in mind the audience - this will help you stay focused on your message. Do use remarks by the troll as springboards for your message, but do not attempt to address remarks by the troll directly.
Internet trolls "cheat" by being irrational (either intentionally provocative, or just naturally insane), so you can "cheat" back by not rising to the bait, and by staying on message.
Make the troll lose his cool, by not taking the bait, and you look better, because you expose the troll and look more mature. This helps you deliver your message, and win the war.