Step one: Ask a friend who'll not be needing to enter a party for a while to help you, make him send an invite to your main character.

Step two: Press alt+f4 to effectively disconnect your main but not kick him out of the party, now log onto your alt and ask a friend to invite your alt into the party with your disconnected main.

Step three: Bring your alt and main to the instance of your choosing using the alt+f4 technique.

Step four: Make your main kill all the mobs that can potentially agro your alt while trying to get him to the boss.

Step five: Now that you've cleared all the mobs with your main, alt+f4 and log onto your alt, make him run straight to the boss.

Step six: Alt+f4 again, log onto your main, and fight the boss…bring him down to about 5-10%, make sure you don't have your pet attack on by then, or dots, etc.

Step seven: Alt+f4 and log on your alt all while the boss is attacking your main, when you log in onto your alt, you will literally see the boss still attacking your main.

Step eight: Make your alt kill the already weakened boss, gather loot, and /dance

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