I know I fixate on the blobs a bit -- pretty willing to raid with thugs/SCs or whatev', but moving a relatively small army into enemy territory is just... hard.
The biggest issue t'me is being able to figure out what's big enough -- being able to take five provinces a turn doesn't matter if five turns later your entire force has been defeated piecemeal and it takes 5+ more to recuperate your losses, all while whatever offered up your defeat in detail rampages around unchecked. Especially given that while a 100 troop 3 mage army may be able to take a province (often easily, depending on PD and defending forces), whatever's left behind probably won't be able to keep it. And while constant raiding is a very, very good thing to be doing (both for the economic effect and how demoralizing it can be for an opponent), it's not, in itself, necessarily going to win the game for you. And the economic effect of it can become very diluted if you're constantly losing your raid groups (of course, that makes the reanimating or domsum nations very attractive for multi-group attacks, since they don't really pay much for their junk).
What's actually nice about the end game is by that point you should (hopefully, and contingent to your nation) have a lot of options for dealing with enemy armies off the main battlefield. Assassins, remote attack spells of both the direct (damaging/assassinating) and indirect (weather/supply targeting/disease, etc) sorts, the as-noted bypassing (if giant army barrels into your nation, hop flying SC/thug(s) a province or two behind them and just clean up after the mess), globals such as vengeful waters, etc., etc. Short of nasty stuff like an SC trapezing in and setting up a lab for an incoming gateway(s), large armies are very... inflexible. Slow, and vulnerable to said defeat in detail if they split up to deal with multi-pronged attacks.
... but yeah, as for actual formations, if it's reached the point the blob is battlefield wide I tend to just say screw it and let it blob. Put casters where they can hit stuff with gift of flight or somethin' or just let the non-caster/unimportant troops get chewed up while the other stuff do their work. At that point I don't really seem them as means of offense so much as ablative HP shields for my actually important stuff, and at that point their placement becomes... less important. I have, however, played a disproportionate amount of chaff heavy nations in SP games, which definitely influences that tendency.