[Tricon]

Marignon has decent mages. In SP I tried a very successful summoning / forging strategy. A beefed up palladin is a force to be reckoned with. The inquisitors are a major asset, but to use them efficiently, you have to understand that dominion is the alpha and omega of dominions 2. They are not just priests with fire magic, they are the means to destroy any dominion based strategy of the opponent.A high inquisitor turned prophet is holy 5, and has access to smite, another very usefull spell (not to mention that you have a regular unit that can constantly cast fanatacism - marignon is very hard to rout) I haven't tried a flaggelant strategy yet, so far I use them fast flankers (which also helps them to keep out of the way of your xbows).
A baphomet pretender worked well for me in the past. He's immobile but very cheap. Fire9 and astral9 is a good choice, and give access to all kinds of nasty spell combos.
One other thing to mention is, that marignon is mayvbe the only nation that has a chance of defeating ermor one on one (SP, haven't crossed swords with marignon against ermor in mp yet).