[DragonRider]

I may be a total newbie in my first ever multiplayer game, but I haven't had any trouble with MA Marignon so far.

Expanding is no problem. National infantry is too cumbersome? Then screw 'em. Don't build any. Produce nothing but casters, crossbowmen, and cavalry (in CBM, Royal Guards are 30/30 and Knights of the Chalice are 80/37). Send your starting army to take adjacent provinces, putting the Pikeneers in front to die (there are enough of them to win your early battles before they're gone).

Then, after a couple of turns, meet up with them with several turns' worth of cavalry and--and I think this might be critical--a couple of Witch Hunters. With an awake Rainbow Mage dedicated to research, you can hit Evocation 3 pretty quick (though you may have to settle for Flame Bolt for a couple of turns before it finishes up) before you start site searching. Two volleys of crossbow bolts plus six fireballs plus Smites wears down attacking independents enough that they'll frequently rout after the first cavalry charge, keeping losses to a minimum. Pick up some Light Infantry or whatever to serve as chaff if you can (they may suck, but they're cheap, don't tie up your capital, and have 2 map move, so they get the job done). Later on, it shouldn't be too hard to get a second group of Royal Guards and Crossbowmen let by another pair of Witch Hunters (all worthwhile Marignon mages and priests have good leadership, which is convenient) to start mopping up weak provinces the main force left behind.

Magic diversity is an obvious concern, but with a rainbow pretender it's not as bad as it looks. Harbingers come with A3, so the ones that aren't thugging can follow your armies casting Wind Guide or Arrow Fend or whatever, and A4 on your pretender or a Ring of Wizardry gets them making boosters. A minimum of W2N1D1 (preferably W3 to mitigate Construction needs) will get you W3N3 and W3D3 as long as you aren't screwed out of water gems. The E2 you no doubt took for Earth Boots and Dwarven Hammers can get you an E3 Troll King. Now you have at least 3 in every path except Blood, and you didn't even have to deviate from blitzing straight for Harbingers to get them. With equipment and communion boosts, the odd E1 and A1 Grand Master can access non-fire evocations, and even the lowly Witch Hunters can manage at least Astral Fires backed with Light of the North Star and communion-propagated Power of the Spheres if you desperately need non-fire damage.

I'm not too sure about long-term prospects, as I have no real late-game experience (I assume fire-reliance gets worse, but angelic summons just keep getting better), but out of the gate MA Marignon seems serviceable enough to my untrained eye.