I've been playing around with this for a few runs now, this has been my best one but i'm not that happy with it. Basically, Muli and Balu will both increase in strength passively over time, so i've been setting it up so the game takes as long as humanly possible to allow them to buff to the max before the bonus round.
The idea is: Iron Man uses Withered and a ton of Luck and Speed buffs to block creeps. Muli is my main carry and Balu is second. Every time a boss, challenge or horseman comes up, Iron Man pushes them back over and over and over again, so Muli and Balu can drink booze and get hugged (and get theirs boosts), but eventually everything slips through.
Here are two screenshots, one of the very end of the game, and one at round 346:
https://imgur.com/a/vKyc0sD
And here are screenshots of the end state at the end of the bonus round:
https://imgur.com/a/d5sMcD2
I'm posting this here because i'm sure there's a way to improve on this, but i'm not getting it. Please let me know if you're able to improve on it.
I have yet to try this, But I think Muli shoul be wearing Spectral set, Seelenreiser, M.reaver and Blade of Darkness.
And Balu should be wearing A Strong Force set, Hydra Arrow, Excalibur and 2 Frozen Heart.
If you make it above 20-40K seconds, you could give Muli Hydra Arrow and give Balu Blade of Darkness.
Muli will get alot stronger than Balu in late bonus round, since he increases crit instead of dmg.
That's why he should be having Seelenreiser for good base dmg% and high multicrit for his crit dmg/rate%