Outbreak Hero

Kill each Outbreak boss at least once.
There are 6 bosses in Outbreak, and the game will let you know when a new boss or enemy type has appeared; the bosses are:

Blaster (this is Brutal Maul from the Story mode)
Invisible Assassin (this is Chaos Slasher from the Story mode)
Chaos Monster (in the mass of Chaos Monsters that come for you, one is considered the "boss" - just make sure to kill them all)
Giant Spider (I've gotten boss awards for killing both the Giant Poisonous Spider and the regular silver Spider, so I do recommend making sure to kill them both)
Skyguard (this is the Skyguard Commander from chapters 4 and 7 of the Story mode)
Shaman (this is the Shaman Boss from chapters 2 and 9 of the Story mode).

All of the bosses will spawn over time, but the Shaman seems to have some kind of mercurial requirement that I admit is completely beyond my ability to figure out - out of around 10 matches split between Brutal Lands and Jungle (avoid Desert like the plague), he spawned roughly half the time, seemingly without any rhyme or reason: he appeared when I died more than 5 times, when I hadn't died at all, when I had only one outpost left, when I had all outposts left, about 5 minutes into a match, about 15 minutes into a match...The only advice I can give for getting him to spawn is to keep playing until he does.

As with killing a human player, you must be the one to actually defeat the bosses in order for it to count as your kill: bosses killed by your troops or outposts do not count towards this trophy.