Ok here you go
The main tactics there was: Turn I: 1. Horsemen positioned back in the middle, "back" is the keyword here, you want to inflict range penalty on the necromancers when they shoot your horseman; ranged units 2 at each side, as forward as possible; 2. Use Soul drain to reduce his horsemen count. 3. Cast Target and shield from the Archmages on the horseman, who defends the position. Turn II - again use defend with Horsemen, and cast Battle Cry (at least on your archmages, can't remember if I had mass); do not use Fighting Trance on mages yet, cause you want to use the shield again on turn 3; Wait with one unit, and when all enemies moved, use time back on the Horseman, cast Stoneskin or Magic spring or Peacefullness on it, and on turn III defend it, then cast Target and new shield from the mage, may be Glot's armor as well.
For the rest of the battle you want to cast time back on the horsemen each time when it is available (do not forget to cast it at the very end of a turn), and keep Target and the shield spell from the archmages (at least).
KIll order - first kill enemy horsemen and inquisitors, then reduce one of the weaker stacks of necromancers (that one which is separate from the other two. After it is reduced you may wait the horsemen once and advance next to the other two necromancer stacks and start tanking them from close, pushing them to use their melee attacks. You want stoneskin + Target on your horsemen at that time.
Keeping Target spell up all the time is very important, as you don't want to allow the enemy necromancers to cast, at least you don't want while there are many of them.
Another secondary goal you want to pursue in the battle is not to allow 3 of your stacks to go together (that's why is the starting position), so he does not use his area damage spells. For this purpose use Glot's armor, Stone walls, Ice thorns, Dispel, or whatever you prefer.
In the best case (which mostly depends on you), you will only lose Horsemen and at the end you will have like 70 horsemen left, you can take a little time to ressurect the dead ones with the inquisitors with timeback, gift, chargers, rage draining, holy anger for extra rage or whatever means you prefer.