what I do to powerlevel my alts requires 3 accounts. I will label them to make it easier account A=Main, B=Character to be leveled, C=person leaving party.
Character C invites A and B, enters queue.
Once in the dungeon C leaves the party, outside of the instanced party A B C will still be in a normal group. C kicks A and B from party
After dungeon, A and B TELEPORTS OUT OF DUNGEON and waits for xp buff to expire.
after the 5 mins queue for another dungeon and enter it. From that point on there will be no more xp debuff and you can run as many times as you want without worry about a roulette