Sacrifice old?! In years perhaps, but not in concept!
Do you have a specific order of missions that you do in order to get certain monsters and spells?
No, I just played 5 times through, once for each god. That's difficult only for Stratos, as his units are so weak. And you'll miss a few events that way.
If you find yourself in trouble, you should definitely get Rainbow from Persephone. That's easily the best spell. If you need more help, you might want to pick up certain hero characters, like Sirocco the Dragon or Toldor the Ent. The fights are a lot easier if you keep them around and alive.
Any strategies in formations/attack strategies/combined arms makeup?
I forgot the formation names, but melee units go to the front, ranged to the side and fliers to the back. You should hotkey each group so that you can assign them targets on the fly and cycle through them to keep the special abilities active. Hotkeys are your friends in general, all good spells need one.
And it's most important to be flexible in your makeup. When your archers are slaughtered by melees, you'd resummon them as fliers.
and I usually go for 2 expensive(3 soul) units instead of 6 cheap (1 soul ) units. Good or bad?
That would depend on which unit we're talking about, I think. Some of higher tier units are suprisingly pathetic. I'm looking at you, Blights.
Do u use guardians at all? They seem useless, since it lowers your fighting force, and any wizard with a couple guys can kill them and convert them before I can reach the manalith anyways. . .
You do use teleport, don't you? With that you shouldn't any problems with the occasional attacks. Guardians are usually a waste of souls, unless you're playing one of these rare missions where the script places you in the defense. Then, guardians can buy you the time you need to regroup.
I usually go for 2 manahours and 2 manaliths, and then concentrate on killing the opposing wizards army and converting, before I finally go after his altar. I tend not to go after his manaliths unless it's on the way. Is that a decent strategy?
If it works... But I doubt you'll get far. Your spells are a lot more potent than your units, and you want to cast them pretty much continuously, even if it's just your basic attack spell. I think you'll run out of mana way too quickly with your setup. Crawling from manalith to manalith and going to 3 manahoars as soon as you can ensures that you have a steady mana supply for your spells and summons.