Hmm… maybe that could help with some of the “too easy” and “too easy for Hammerdins” complaints.
Give the guest monster an immunity. There would need to be some kind of predictability about it though. Maybe make it a specific guest monster in each Act? That has a specific immunity? Basically, which ever guest monster is sent to help Terrorize the zone has an immunity suited for that zone?
For example, Act IV it makes since to have a guest monster that is Fire Immune, since it’s the Burning Hells.
Act V it would make sense for the guest monster to have Cold Immune because it’s the mountains, and it’s got all those icy areas.
Both Act II and Act III kind of have poison themes going on, so let’s make the decision based on where there already are immunities. Act II has a lot of Poison Immunes already, while Act III has a lot of Lightning Immunes. On the other hand, Act II has the Scarabs with all the charged bolt. Well, it means one of them should have a Lightning Immune guest monster and the other a Poison Immune guest monster.
That would leave Act I getting the Magic Immune guest monster. This would immediately reduce the benefits a Hammerdin gets from say a Terrorized Moo Moo Farm.
But what should the guest monster be? Maybe two possible options to spice it up a bit?
Act 1: Magic Immunes: Overseer, Claw Viper
Act 2: Poison Immunes: Oblivion Knight, Zakarum Priest
Act 3: Lightning Immunes: Scarab, Slinger
Act 4: Fire Immunes: Goatmen, Stygain Fury
Act 5: Cold Immunes: Giant Mosquito, Council Member
Oblivion Knight and Zakarum Priest make sense for Act 2 because of all the undead. Overseer fits Act 1 with the Fallen and Cows, and all the cannon fodder, while Claw Viper sort of fit in the woods/moor theme. Scarabs and Slinger both fit in the jungle. Goatmen and Stygain Fury both fit Act 4 and the Hells.
Act 5 wasn’t so much about fitting as find something that wasn’t already in Act 5 as a guest monster. Mosquitos, Swarms, and Council Members are all I could come up with. I guess Mosquitos fit a battlefield, and Zakarum Zealots/Priest are already in Act 5, so maybe a Council Member or two can be there.
Something like that anyways. Act 5 already has a ton of Cold Immunes, same with Act 4 and Fire Immunes, so really this ends up making it more difficult for Magic type damage, but not so much that Hammerdins, or Bone Necromancers would become unviable.