I've been making a few homebrew monsters for my current D&D 5e campaign, and I ran into a problem when I wanted to create a creature that used healing magic. There were general rules for offensive spells in the DMG, but none for healing or utility. I've looked around at several RPG forums and discussion boards, but all that I found just talked about offensive spellcasters or the monster didn't really act like a healer. The latter instance was actually on this forum (How do I calculate the CR of a monster that spends their turns healing?) where the monster was a cleric, but that cleric didn't do any healing in the first three rounds of combat. All they did was cast buff spells, so that doesn't answer my question. I'm hoping someone can shed some light on this issue for me.
Here's a stat block for the creature I'm wanting to make:
Medium ooze, chaotic neutral
Armor Class: 15 (natural armor)
Hit Points: 120 (16d8 + 48)
Speed: 30 ft.
STR 11 (+0)
DEX 13 (+1)
CON 16 (+3)
INT 12 (+1)
WIS 17 (+3)
CHA 9 (-1)
Skills: Medicine +5
Damage Resistances: Acid
Senses: passive Perception 13
Challenge: 3 (700 XP)
Innate Spellcasting. The slime's innate spellcasting ability is Wisdom. The slime can innately cast the following spells, requiring no components:
4/day each: cure wounds, healing word
1/day each: aid, lesser restoration, mass healing word
Whap. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 3 (1d6) bludgeoning damage and 2 (1d4) acid damage.
My main question breaks down into two parts:
If the monster primarily uses their action to heal, how should I calculate their damage-per-round? I would assume that I would just use the most healing & damage possible across the first three rounds of combat, but I'm not sure if healing is treated different from damage when calculating damage-per-round.
How should I handle calculating the average amount of healing for the spell mass healing word for the CR calculations? Since it can target multiple creatures, I would assume I have to multiply the average value rolled, which is 5.5 (1d4 + Wisdom), by some number to account for multiple creatures being healed at once. Do I treat it like the spell fireball or a dragon's Breath Weapon and just multiply it by two, or should I multiply it by the max number of creatures that can be healed by this spell divided by two, which would be three?
I realize that sometimes it just has to come down to play-testing, but I feel like this is something that could more-or-less be calculated without any guesswork.