You do not need to kill Omnitron Defense for this, so you can spend some time in their heroic mode after Magmaw is down.
As you all know there are adds(Blazing Bone Construct) which spawn during the whole fight in Magmaw hardmode. But as of now you do not have to deal with em. You need a paladin holy (or any class that runs higher than 100% speed, can taunt and heal itself) and 3 hunters.
Paladin will taunt first Blazing and will run down stairs, heading to Omnitron's way, but he'll take the other stair instead of heading to Omnitron Defense itself. Hunters will take turns as Blazing Bones spawn, heading to the kiter, then using MD and Feign Death after the mob is taunted. This way, you only deal with parasites and other mechanics, ignoring those adds.
Paladin is the best for this as he can use Consectration and make aggro on all Blazing Bones as close to the final there will be around 16 or so Blazing Bone Constructs.
First topic here, so idk if this should've been posted in "Guides" section as i believe it's not intended for us to use 3 rooms to kite mobs from a boss and they'll probably fix it soon.