A 1st level dip into fighter gives casters armor, shield, and con save proficiencies. If that's not enough then a feat can give you advantage on concentration saving throws. So a +2 becomes something like a +6 and then you could get two chances with that modifier.

If you start a game as a caster, as an origin character for example, then you can simply go to withers and reset your character to a level 1 fighter and then just take all your spellcasting levels.

You have to have a really high AC, which is really easy to get for Shadowheart/Cleric with all the heavy armor and other healer gear available. Add the Blood of Lathander mace, and you are set.