Oh Novos!

For soloing:

Neither the undead adds from the top of the stairs nor the Crystal Handlers from the sides are allowed to get close to him. The undead run towards the boss, while the Crystal Handlers run to the player. Stand at the top of the stairs and intercept all undead as they spawn. The Crystal Handlers ("bolster my defenses, hurry curse you!") will run directly to you, and if you are at the top of the stairs then they will not get close enough to the boss to fail the achievement. If you stand too low on the stairs, then the Crystal Handlers will run near the boss on their way to you, so stand all the way at the top.

Track the achievement. Once you've killed four of the Crystal Handlers and phase 2 begins and the achievement tracking letters are white (not red), then you're home free.

Don't let the bodies hit the floor!

Simply stay on the Stairs and kill the Mobs. There will be NO more adds spawn after Novos shield brakes down and you engage him. So you dont need to pay attention to that anymore.