Hey! a little bit late on the response, sorry haha
Yup, I agree with mortician being a good class to have as a mercenary:) you can also carry around corpses and drop them when you need them, as husks will decay after a few minutes.
About runes and spellcrafting... it all depends in what's your strategy for your run, in your case (3 spell-slot, caster class) I would go for:
- (X/Y) Shield of Rejuvenation: X and Y being the elements you want to focus on. Self-castable spells will always trigger their effects, so they are good to buff yourself before a battle or to heal after one. Also, the shield spell boost all of your attunements, which is good if you are looking to make the most out of your elemental effects, attunement also (maybe?) boost your elemental damage but I haven't tested that yet. The Rejuvenation lasts for about 6 turns with Average charm, probably increased with spell quality too
- (X/Y) Curse/Field of Vulnerability: Curse if you prefer going for single target damage, Fields if you prefer AoE. The good thing about vulnerability is that apart from increasing physical damage taken, it also increases the duration of other debuffs.
- (X/Y) Barrage of Poison/Blinding: Barrage spells won't trigger elemental effects, but they will benefit from the bonus damage from curse and fields, and at higher spell quality the AoE is huuuge. The effect will be longer due to the vulnerability, Poison is good if you don't have another way to inflict Poison to counter healing enemies (i.e an Earth Staff), but you could change it to Bleeding for enemies who dash a lot. If you want to go for a more defensive effector, Blinding is one of the best for that.
It's advisable o diversify your spells' elements, just in case you find a strong enemy resistant to the attunement you are trying to go for, the arrows spell are also good for that, as you said.
In my experience,stat-buffing effector runes are stronger early in the game, when the bonus stats are pretty noticeable (probably even better than a single masterwork piece of equipment) , and they are amazing if you are going for high reputation with the Radiant Hand to get the traits that makes buffs better, they fall off later into the game when you reach Godlike in your needed stats, though they could be good to suplement your weaknesses. You are correct about stat-debuffs being a little situational, my advice here is figuring what type of enemies you are struggling against the most and then deciding if you want some stat debuff to counter them, keep in mind that High Resilience enemies will make your debuffs almost useless, so it's good to have vulnerability just to decrease a little more their thoughness.