Myrmidon stuck "underground"

So I was defending an ancient forge from pandoran attack. I started on high ground about 2 to 3 “stories” above base level terrain. The pandoran attackers come in on on the low terrain opposite. Finally, down the left and right sides on the map are multiple sets of stairs and platforms. The pandorans were otherwise defeated with only one myrmidon remaining. The myrmidon came at our party up the right side set of stairs and “disappeared” into the terrain on the way up. It continued to bounce around the bottom of the sky box underneath my party. The mission could not be completed due to this last enemy, although in hindsight I could have moved everyone to the low ground to lure the myrmidon out back into the normal areas of the map.

Mods: I meant to put this in bug reporting, can you move TIA?

If you have a save of this situation then you should load it and use the ingame report tool by pressing the F10-key. This way the devs will receive your save- and in-game status and probably can get a solution for this bug.