Easy peasy, a stack of potions for defense, a stack of buffs for defense as well, initiative, and accuracy. If you have paralyzing units, the Guardian becomes just a sturdy puppet.
Against the undead with upgraded warrior ghosts (3 in the first row), you don't need any special tricks to take down a single Guardian in the capital.
Everyone else has damage and some perks like the witch's morph (second branch of mages).
Also, elemental damage-DOT doesn't depend on defense and deals a fixed amount of damage to any target. And the Guardian has no defenses or immunities.
And in general... for a very well-leveled army, killing a Guardian is not a problem.