* Multiplayer is a good way to earn MSP and a few medals when you run out of Sortie Points. But the matchmaking is a total crapshoot, and battles usually come down to who owns the best premium units, not skill. I advise avoiding multiplayer for the most part.
* Emergency stages are limited-time stages that appear for one hour daily at noon, 6pm, and 10pm Central (-0600 GMT). Rescuing all of the POWs in these stages will permanently unlock the stage as well as some fairly good units.
* The daily Emergency levels (found on the green map) offer seven unique units to win as well, but those levels are much harder than normal. Save them for when you've unlocked most POWs and built your strongest deck.