This will require another player on your friend's list to assist you.
A will be the person killing Izual and B will be the person assisting A in killing Izual.
B will create a game and A will join.
A and B will need to progress through Act IV till they reach the Great Span.
B will then set the game to public, leave the game and click resume game.
A will then enter the Great Span, kill Izual then join B's game through Quick Join in the bottom right.
A will then click B's Banner to teleport to the Great Span.
B will then set the game to public, leave and click resume game.
A will then enter the Great Span, kill Izual and join B's game through Quick Join in the bottom right.
And the cycle continues.
(I'm sure some of you will be able to put this same method to better use else where.)