And it is in fact almost imperative to keep few units in front of the Lord - every time the Lord attacks, your Castle loses 500 HP. No matter where it is.
The Lord of Hell is rather easy - it performs no attacks if less than, say, 3 or 4 units are in range wailing at it. ie. All Titans. Since it takes 4x damage from Melee and only about 1x absorb from magic, it is not too amiss to keep around some ranged units (you probably have some mages left over from killing the castle anyway) to shoot down enemy melee so they don't get to take a swing at your Titans. Given that, the Lord of Hell is just a long fight, a lot easier than the End and the Overlord.
Upgrades don't reapply when you randomize a creature: ie, the Magical Creatures gain an Action upgrade does not apply to newly generated Magical Creatures. I am also unsure if the MAX Level action upgrade also behaves in this way - not very willing to find out.
 "Epic War 2"){kind=icon}
 "SeppuKuties"){kind=icon}
 "Brute Wars"){kind=icon}