The problem with Plague is that he is more of an anti-troop than an offense troop. It can be fun progressively withering entire enemy teams with his trait, but now that Kraken exists, if you want passive looping damage that is the better choice. Last time I used him, there was still the bug with his third trait where it would occasionally prevent a four or five match of skulls from killing the enemy as well as failing to kill the enemy if the trait fired on more than one stat due to a bunch of simultaneous matches (eg. when using a converter). Not sure if either has been fixed.
I've had some fun with these:
Khorvash, Valk, Mercy, Plague, Lion's Banner
Loop Valk and Mercy back and forth to stun and wither the entire enemy team. Even with just armored, Khorvash can easily take a few skulls with Mercy backup. Don't bother casting Plague unless the enemy has no good moves. It is very rare that an enemy will get a spell off versus this team with the combination of mana drain, disease, and looping.
Crimson Bat, Seer, Spider, Plague, Green/Purple banner
The purpose here is actually skull damage. You can use a mercy/valk loop, just like previously, but they are a bit harder to loop to get mass skulls. Basic premise is that you use the combination of converters to perfect loop between each other, and once you have cycled through all colors, you end up with a bunch of skulls. Once Aspect of Plague hits life even once, Crimson Bat permanently does double skull damage to every troop. You can use mass converts to eliminate other colors of gems and get the skulls you need for Crimson Bat, or to fill his mana. It never was really all that fast, but I pretty much stopped using with the prevalence of stuff that can instakill, stun, or entangle your first slot troop if it is not impervious because it renders most of the team moot.