There are two main ways to protect your brother or sister in arms against Diablo in normal. Either you must keep the distance of both of you while still somehow dishing out effective damage. A sorceress can do it with frantic teleporting but even then it is a tall order.
The other path is equally extreme, but more reliable for the hero that can stomach it. You must get close to the Lord of Terror and wield such superior weaponry dealing so much more damage than your comrade, that you appear as the indisputably bigger threat. Needless to say, you need to stand in another direction so that the devilish spells do not accidentally wash over your ward.
Ranged mercenaries are slightly easier to keep safe if they lock on to the demon and fire their arrows or spells but their bad habit of following their employer closely can lead them to wander into Diablos sight. Melee mercenaries need to get close which is a danger in itself and run the risk of ending up next to the employer when Diablo appears. Should potions run dry, it is also far harder to disengage successfully when the mercenary is next to the enemy.
At level 34 and with the best equipment that untwinked un-magic-finding and item-restricted gameplay allows, the ANTS prepare to try their luck, or lack thereof.
Celexia is first to engage Diablo. In a display of unprecedented genius, Elexa stays out of the way and sight and keeps shooting but not moving. With Celexias rapid kicking, the day is decided in the ANTS favour and the Lord of Terror futilely clenches his hands at the humiliating defeat of the first round.
Nymia, while lacking serious hardware, nevertheless manages to dish out tremendous lightning damage and strike fast enough to overwhelm Diablo, with Devak the dodging doing quite well as well and chilling Diablo while chilling out at the side.
Ignatia faces a slightly different situation as her main damage source are traps and not attacks. She needs to balance between tossing out wakes of inferno, cursing with her wand of life tap and striking with her jewel-boosted wrist blade. All the while Scorch the Scorched earns his name as fire novas blacken his robes. It is a hectic and close call but Ignatia holds her ground long enough to serve a well cooked soulstone salad witht he same procedure as last year, after passing a convenient gem shrine where ingredients for the side dish can be harvested.
Last, with slothful weapon types and a meagre elemental attack combined with the dumbest life form of Sanctuary and possibly the universe and multiverse as a whole - the Act II town guard - Octavia braves the danger of demonic proximity. While she is also aided by a life tap wand her melee attacks are mainly elemental and not physical so the leeched life is less than for most combat characters. Zanarhi, disarmed so as to not attract Diablos attentions, offers little assistance. It is the most even battle of them all and only after a dangerous retreat to buy potions and many frustrating interrupted attacks and underwhelming hits does Octavia prevail. But prevail she does and Zanarhi remains alive. Rarely have the sight of the withering red demon been more welcome.
The ANTS have stood their ground in the burning hells without casualties, which is no mean feat for an untwinked character, and unlocked the promising paradise of Act V:s experiences and upgrading opportunities. Onward towards the north!