Basically what it says on the tin. If a team really has their stuff together and clears all 5 rounds in 17 minutes or less, (That’s 3 min round average, plus the 4 30 second intermissions) you could potentially reward them with a super-nasty bonus round where all the stops get pulled out and you throw everything you can think of at the players. Phantoms, Scarabs, squads of Goblins, Wardens that can fly around, you name it. If the team manages to somehow overcome these shenanigans, they earn a hefty extra chunk of loot! If they fail the bonus round, they still get rewarded for clearing the game as normal.
Your bonus round sounds like a nightmare rather than a bonus. I like the idea of a bonus round, but not something overly difficult. More along the lines of a grunt party wave where you have to kill 200 suicide grunts or something along the lines. Maybe even turn on grunt birthday party.