Combine one or more of these with an iron golem and have them cast heat metal on it.

Ultimate destruction! I love it!

With these guys summoned, Conjure Minor Elementals just becomes Mass Heat Metal, which is amazing. The low save DC hardly matters - the damage comes without a save!

I was running this monster for a party of 1st-level characters the other day, and 2 of them were martial classes in chainmail.... just shows how the CR system is really broken; theoretically an encounter with CR 1/2 should be trivial to even the lowest level party, but with the completely OP direct 2d8 damage EVERY ROUND from the heat metal spell, no saves, no attack rolls, this monster can one-shot the party's tank (fighter/paladin). If it keeps its concentration and rolls moderately high damage, not even healing can save a character from permadeath. Of course on its own, it usually wont result in a TPK but it'll be real devastating for that person who lost his/her character on the first session. Not to mention when paired up with other monsters it can provide a serious threat to even higher-level parties. Just beware of monsters like these who can punish specific characters to the point of death while being an "easy" encounter for the rest of the party, it can seem unfair and purposefully targetting one person if you're not careful with it.

Quote from BabyKangarooACS >>

I was running this monster for a party of 1st-level characters the other day, and 2 of them were martial classes in chainmail.... just shows how the CR system is really broken; theoretically an encounter with CR 1/2 should be trivial to even the lowest level party, but with the completely OP direct 2d8 damage EVERY ROUND from the heat metal spell, no saves, no attack rolls, this monster can one-shot the party's tank (fighter/paladin). If it keeps its concentration and rolls moderately high damage, not even healing can save a character from permadeath. Of course on its own, it usually wont result in a TPK but it'll be real devastating for that person who lost his/her character on the first session. Not to mention when paired up with other monsters it can provide a serious threat to even higher-level parties. Just beware of monsters like these who can punish specific characters to the point of death while being an "easy" encounter for the rest of the party, it can seem unfair and purposefully targetting one person if you're not careful with it.

 You should see the article Beyond just posted on Heat Metal.  It has good advice on how DM's should use the spell.  Using it to heat weapons or other metal objects the can be removed quickly will make it seem fair when the characters have no means of countering this spell (counterspell or dispel).

thank you for you build and staff

This stat block suggests that when the magma mephit casts heat metal, there's a spell save, but the spell doesn't require a save, normally at least, right?

it does

Oh you devious little devil.

would it be wise to capture one of these to take as a heat or powersource for a proto-spelljammer?

This stat block suggests that when the magma mephit casts heat metal, there's a spell save, but the spell doesn't require a save, normally at least, right?

The con save for not dropping the hot metal.

My god, this thing is AMAZING.

Im planning on using these guys in a volcano themed campiegn. Should I ditch them or not? 

These things are 100 percent the best type of mephit

Good low level Encounter

Love this description....I'm borrowing it, hope you don't mind