Callagadra can be the toughest challenge, moreso than Crate, for many builds due to her high damage and tankiness.
The general plan most people use for killing Callagadra revolves around kiting away from her constantly and using ranged attacks, caster skills, or pet ressummons to chip away at her.
Many ranged builds (with enough defense to survive being one-shotted on crits/layered attacks (from pets) -> good armor/physical resist, ~/+ 12k hp, ~/+ 3k DA) can kill her after you learn about the fight/mechanics and play the fight well.
If you’re looking for a build to face-tank/fight Callagadra up close, here’s a link to one of the safest melee builds (and it has a good kill speed): Conjurer - Vitality 2H
Other melee Callagadra builds I can think of are: Retaliation Warlord (stacking armour and/or block), Blade Master (Belgothian set/stacking armour), variants of Conjurer/Ritualist (all using Dark One’s set)
But IMHO the build I linked you has the easiest time and is the most reliable (doesn’t need to buff with/use consumables and more forgiving for errors mid-fight)!
I wish you GL friend!
edit:
Here’s a ranged build that has a really easy time kiting and killing Callagadra:
Druid - Lightning Storm Totem
(this is a general build and you will want to tweak it for each boss, e.g. change item/components/augments to overcap specific resists and move devotions around to make it easier to kite, e.g. move RR debuffs to totems/wind devil instead of active skills)