Is the earth elemental's Earth Glide trait as potentially game-breaking as I think it is?

Question

I'm DMing a campaign themed around monsters based on the elements, so making use of the Elementals naturally came to mind.

Taking a look at the Earth Elemental, it strikes me that there is some very strong potential for making use of incredibly cheap tactics depending on the terrain.

Essentially, the way I see things is that if the Earth Elemental is burrowed, weapon attacks have no way to attack it, and because of the Earth Glide move, it makes sense that the PCs would lose track of the Earth Elemental meaning that when it does attack, it'll always be able to surprise the PCs and attack at advantage.

Beyond that, because Burrowing does not constitute an action in itself, there's nothing stopping the Earth Elemental from popping out of burrowing, getting two slams in (one with advantage), before burrowing (admittedly, risking a possible opportunity attack), and thereby depriving the PCs of the chance to attack it.

I'm just wondering if there's something I missed in the rules to make this tactic unviable, as I see it as a potentially game-breaking encounter for a group, especially one with limited spell-casting options.

Answer 1

... if the Earth Elemental is burrowed, weapon attacks have no way to attack it

Correct.

... because of the Earth Glide move, it makes sense that the PCs would lose track of the Earth Elemental

Incorrect. Unless the Earth Elemental takes the hide action, its location is known. The fact that it can move through the earth without disturbing it doesn't mean that it can do so without giving its position away - there may be visual, audible or other sensory clues as to where it is but however you decide to narrate it, something reveals its location.

Unseen is not the same as hidden. See What advantages does hiding have?

... when it does attack, it'll always be able to surprise the PCs

Incorrect. You can only be surprised at the start of the combat. See How often during combat can you be Surprised?

... and attack at advantage.

Incorrect.

First, being surprised does not give the attacker advantage.

Second, while unseen attackers get advantage, however, the Earth Elemental must leave the earth in order to attack and is therefore not unseen when it attacks.

... there's nothing stopping the Earth Elemental from popping out of burrowing, getting two slams in (one with advantage), before burrowing (admittedly, risking a possible opportunity attack), and thereby depriving the PCs of the chance to attack it.

Correct. Apart from the advantage as discussed above.

I'm just wondering if there's something I missed in the rules to make this tactic unviable,

See above.

... I see it as a potentially game breaking encounter for a group, especially one with limited spell-casting options.

Game breaking? Hardly.

A potentially problematic encounter that requires the PCs to develop novel tactics on the fly? Sure, but that's a good thing.

One strategy that my players used was to simply Ready (ranged) attacks and spells triggered "when an Earth Elemental leaves the Earth." 4-5 PCs wailing on a single Earth Elemental each time it sticks its protuberances out soon turns it into gravel.

Answer 2

I don't see any reason for it to not work, on the other hand, I don't see how it is broken either.

First off: yes, fighting an Earth Elemental in a completely earthy terrain with no worked structures gives it a good advantage. So does fighting a Water elemental (or even better example: a Water Weird) underwater. And yes, some creatures are a pain to defeat without proper spellcasting. The reason being: most monsters and encounter guidelines were probably designed assuming a somewhat balanced party. If you try to fight a Black Pudding with nothing but Swords will get you TPK'd as well.

But I am not even sure this is the case here.

Readying actions is a thing.

So, the Fighter with +7 in Athletics says "After the earth elemental shows up and attacks once, I will grapple it". There you go, speed reduced to 0, Earth Elemental can not escape. The Earth Elemental is only Large, so it can be Grappled by any Medium creature. Bonus points if anyone casts Guidance on the Fighter, or, even better, it's a Raging Barbarian that gets advantage on the check.

Cheap works for both sides

The Wizard casts Levitate. Being a Divination Wizard, they force the Earth Elemental to roll a 1 in its Saving Throw. The Earth Elemental is now effectively useless because it has no ranged attacks, so the party will just attack it from range and kill it.

About the Earth Glide = Losing Track thing

I'm not too sure about that granting insta-advantage. Usually, you would need to Hide in order to do that. Otherwise, the monster is not exactly trying to, well, hide itself and be sneaky on its attacks. With its amazing -1 Dex, I doubt it is quiet enough that the characters who are actively paying attention wouldn't notice it coming out of the ground before attacking.

The Monster has a Low Int score and just average Wisdom

And finally, the Earth Elemental has a whooping 5 Intelligence and 10 Wisdom. I'm not sure it would even behave like that. Most likely it would just get enraged and start slamming the PCs or something irrational like that. While they are described as ambush hunters, ambushing and hit-and-run are quite different styles, and, in my interpretation of the scores, understanding that the puny-looking humanoids that are 1/4 of its size are actually dangerous enough that it needs to be constantly hiding and "outsmarting" them requires more than 5 Int in my world.