I'll throw out how I like to organize my army:
Manahoars in phalanx formation.
Ranged units in line formation.
Melee units in skirmish formation.
Each group is their own control group so when I'm getting set up, I hear:
"You are group 1, skirmish formation, guard me"
"You are group 2, line formation, guard me"
"You are group 9, phalanx formation, guard me"
Flyers tend to be group 3 if I use them and either skirmish or line formation depending on whether they are ranged or melee.
During battle, it's good to select the groups in turn to see how they're doing on the panel on the left side of the screen. You can heal your creatures by targeting them in that panel as well, which is way easier than finding them on the map.
I will sometimes send my army away from me so I can go cast a spell or run ahead, but I rarely split the army apart itself. Guardian is quite useful though I never use it on my alter itself for campaign missions, that's just not close enough to the battle to be useful.
Speaking of the map, you can middle click drag to zoom it, I like it all the way zoomed out so I can see as much of the map as possible. The reason you don't need to split up your army is that you can always teleport to your structures, just make sure your army is gathered around you so that they all come with you.
In battle it's also usually best to focus on the enemy wizard since once killed he can't pick up his own souls so you can then convert them. If his manahoars are alive, he can use them to regenerate, so watch out for that as well. You've got to be careful though since if the enemy kills of a bunch of your units, your wizard becomes more exposed. Always keep an eye on your health and try to keep the armor spell in use whenever in battle.