This probably isn't a particularly practical technique, but it is hilarious. I started a game on Crucible with flithy rich starting cash and level 15 starting level, purchased the highest rank of Mist Form, Bat Swarm, and all the Night Walker upgrades. I used my starting cash to stock up on helms - basically bought all the ones I could afford, although I'm sure that was substantial overkill (I still haven't figured out the math on mana regen). I then used Bat Swarm to teleport on top of the enemies portal flag, hit the mist form button, and sat there.
Mist Form killed off a few of their minions, who convinced a few more to stay a while and die. I very quickly had turned their portal into my nightwalker production facility. I took out the two nearby towers, and sent my night walkers to go hammer on the citadel.
This presented my oponents with a difficult choice. I myself was unkillable, with enough mana regen to remain in mist form indefinitely, costing them a portal at the very least. They could port back and protect their citadel from my nightwalkers, but then they were down a portal and a hero. Or they could let me keep beating on their citadel, and risk losing the game, while pressing a one hero advantage in an attempt to seize my citadel first. The computer AI was extremely confused by this tactic, as one might expect. End result? Hilarity.
Against a real oponent, I suspect this could be countered by sensible citadel upgrades. If the minions coming out of the portal are strong enough to survive a few seconds of Mist, then the initial minion generation would require I shift out, leaving me vulnerable for a few seconds. Furthermore, in a normal game, I'd be badly gimped early on by the bizzare (for Erebus) gear choices.
Of course, there're additional twists possible. One is to use myself as a form of improved Portal Lock - I can shift out and seize the portal myself, generating real troops. In an actual real late-game, with catapulasauri, this'd probably be a superior tactic. Inifinite mistform can have quite a few other uses, as well. No other class has /permanent/ invincibility.