[Shovah32]

You probably need to get more mages and research more, late game mages, summons and SCs dominate the battlefield so try some of the following:
Research construction and forge boosters for your mages to cast big spells

Research conjuration/enchantment to get summoning spells, combined with construction this can give you access to thugs/SC's(summon black servants, spectres, banelords and maybe even wraithlords/tartarians) and equip them either to solo enemy armies or support your armies

Research evocation for battle magic(iirc you have fire which is obvious for artillery, you can also get things like shadow bolt, banefire, maybe even nether darts) which can also go well with conjuration(phoenix power ect to boost skill level in battle) or construction (forged boosters).

Research enchantment for skele spam, if you have d2+ mages(iirc you do) they can cast raise skeletons. While a single mage casting this isnt impressive, a bunch of them can have a real effect on the battlefield.

Research alteration/enchantment for troop buffs, casting spells like luck and body ethereal on tough troops(sacred cavalry, principe ect) can make them alot tougher, as an example with both of those enchantments on a group of men only 1 in 8 hits will effect them(ethereal stops 3/4, luck stops half of the 1/4)

Take an awake/sleeping SC pretender(any tough pretender who can cast some buffs on himself/solo indies without buffs)

Try a bless strat, f9w9 makes your sacreds both survivable(shield, 21 def, 13 health, recuperation), fast(+50% movement) and hard hitting(14atk, light lance, hoof, 50% chance of extra attack, all attacks flaming)

You could use any combination of these or plans all of your own, telling us your strategy would help us in helping you.