If you think the Capital Guardian is tough, wait till you meet some of the End-Game bosses.
Taking out the Captial Guardians is as easy as pie. Different races call for different tactics of course.
The Empire can use the Prophetess in conjunction with high armour(potions, spells, banner). Dead simple.
The Legions of the Damned have the Incubus. Besides the mandatory armour, give the Incubus potions that increase his chance to hit until it hits 100%.
Mountain Clans have high HP, ungodly damage and Frostbite from the Son of Ymir.
The Undead Hordes have the Shade.
General guidelines,
1) Strive to give 90 armour to all your party members.
2) Try to hit the damage cap.
3) Use Poison, Blister and Frostbite abilities.
4) Use Paralyse and/or Petrify when possible. This includes artifacts.
Have fun.
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Warning: Capital Guardians give 8000XP, so try not to waste it.