Fire Elemental

A fire elemental on the party's side almost always belongs to a summoner or a bound contract: a druid's Conjure Elemental, a wizard's planar binding, a genie patron's loaner, a pact made at a forge that's older than the city. The elemental does not converse, it speaks Primordial (Ignan) and nothing else, and it understands "burn that" and "stop". Treat it as a weapon with feelings rather than a teammate.

Position is the whole problem. Fire Aura does not care about friend or foe, so any PC within 10 feet of the elemental at the end of its turn takes 1d10 fire. Tell the party once. After that, anyone who stays in the aura is making a choice, and the cleric's healing budget should reflect it. Send the elemental into the densest enemy cluster and let it stand there. Two Burn attacks at +6 for 10 fire each, plus aura, plus the on-fire rider, will chew through any unarmored mob in two rounds.

The elemental will not pull punches and it will not retreat to spare an ally. If a PC drops in melee next to it, the aura keeps ticking on the corpse, on the cleric trying to heal them, on whoever else closes to help. The summoner's only real job is to point it somewhere and accept the collateral. When the binding ends or the spell drops, the elemental leaves immediately and without ceremony, often setting the room on fire on the way out.

Have the summoner give the party a one-line warning before the elemental arrives, then let one PC eat aura damage on round one. The lesson sticks better than any speech about positioning.

A fire elemental is a melee mob that punishes anyone who stands next to it for the crime of being flammable. Fire Aura ticks 1d10 fire on every creature within 10 feet at the end of the elemental's turn, and Fire Form means it walks through party formations and lights the wizard on fire on the way past. The encounter is a positioning puzzle wrapped around 93 HP and AC 13.

Open by closing distance, since the elemental's only attack is melee. With 50 ft. of walk speed and Fire Form, it can split a 1-inch gap and end its turn between two PCs to maximize aura ticks. Multiattack is two Burn at +6 for 10 fire each, and the rider sets creatures and flammable objects on fire. The on-fire condition keeps damaging targets even when the elemental is engaged elsewhere, so don't waste the opener on the tank in plate. Aim Burn at the bard, the rogue, the wizard, anyone with cloth or leather and a low touch-AC to ignite.

Use the terrain. Drop the elemental into a wooden tavern, a hay barn, a tinder-dry forest, a ship's deck, anywhere the secondary fire damage compounds. Resistance to Bludgeoning, Piercing, and Slashing means the front-line martials are doing half-damage even when they hit, so the fight runs longer than the HP suggests. Fire immunity shuts off the warlock who picked Eldritch Blast with Agonizing and the sorcerer who only ever casts Fireball.

Water Susceptibility is the kill switch the party will find if they're clever. Five feet of water movement is 1d6 cold; a thrown bucket is the same. A creek crossing during the fight should drop the elemental's HP visibly, and a clever player who decants a waterskin onto it deserves the win. Once it's below half, it bull-rushes the most-damaged PC for one last aura tick before the math runs out.

Describe the heat haze before initiative. The party should feel the temperature climb before the thing rounds the corner.