(DnD 5e) Cheap Earth Elemental tactics

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.