Chaff up front. "Real" troops and archers in some kind of NOT-blob behind them (leave some room for overshot arrows/evocations). Some rearguards in case of fliers/battlefield summons (Howl etc.). I usually leave my mages in a long line, either filling up squares or just 1-mage/square, depends on my numbers. Slaves, enchantment casters and long-range evokers get placed far back. Others generally get placed in the sides or rear of the "real" troop formations. If possible try to have some big bags of HP in your chaff (trolls etc.) as lightning rods.
Idea is that the chaff gets shot up so that the better troops can advance relatively unharmed. Shots aimed at the better troops will hopefully hit them and not the mages in the edges of the formations.
Getting some buffs on your mages really ups their survivability. S9 bless, battle fortune, wave warriors, anything you can cast. Really easy if you have communions or battlefield wide stuff available.
It's definitely tricky, I'm not honestly sure I'm even doing a great job of it.
I guess the easiest way to un-blob your army would be to divide it. Break your army into smaller splinters if you can get away with it. Instead of taking province per turn from the enemy, take three or five. Hundred guys and some mages x3 gets more done than three hundred guys and a blob of mages. Admittedly that depends on the relative strengths and the geography of where you're fighting. I've seen some players (on bay12 and elsewhere) to get reeeally fixated on big blobs. There's a time and place for those, certainly. Can't break a castle with fifty dudes. But being flexible enough to hit multiple targets at once can be a great income denier for your enemy.
Best way to fight an unholy lategame doomstack is to go around it if you can. Either that or turn-1 earthquake