Here is how I would evaluate it, and much of this may be wrong! But, it should give me(you) an idea of how the battle will be.
Compare Att and Def of the units that will be in melee and look up the DRN to see chance to hit. Make an approximate adjustment based on numbers and size (units/square) of each side and which side will end up with extra attacks on it per unit (reducing defense). C’tis infantry have mostly Att 10 and the Forest Trolls 9, so you will be hitting them baseline 54% of the time. Forest Trolls have Att 10 and your infantry has 12-13, so that would be a 24 or 30% chance for them to hit you (plus you need to add in the shield parry which helps you more). I would adjust this more in your favor as you should have 20-25 units and they only have 5-9 probably, so you will surround them causing defense reduction per hit (same in Dom 5?) and your 3 units per square vs their 2 will skew this even more in your favor. You may even end up hitting them well over 2-3 times as much each turn.
Compare damage numbers and protection. Factor in special abilities (regen here). This is where your problem comes in. I haven’t reviewed the formulas recently, but you do compare the damage of each unit to the protection of the other and add a DRN of damage to each. But, to get you an idea here, most of your spears with your strength give you a base damage of 13 vs the Forest Troll protection of 12. You get the DRN adjustment, but you will not be doing much damage here and will be relying on lucky differential rolls of the DRN (spears get 20% piercing to prot, though, so some help here). in Dom 4, the Forest Trolls have a weapon damage with str of 26 vs your protection of 5, 12 or 16 depending on the unit. These guys will be one shotting you a lot unless you get your shield to protect you, and even then it may not be enough as Two hand weapons now get 125% of Str bonus and blunt weapons do more damage on head hits and do more shield damage.
Compare morale of units going against each other. I really need to review morale check rules, but once morale checks start happening, it is good to compare which units are harder to break. Forest Trolls are Morale 11 and your units seem to be Morale 9. This isn’t good for you at all, however, if you made a commander a prophet and have access to Sermon of Courage, and have a good leader commander to boost morale, and fight in a province of your dominion, you may be able to swing this slightly in your favor.
Based on the above, I would estimate that you may do similar damage to each other each round overall, but with the trolls bigger health pool, and regen, and higher base morale, you may lose out. If you win, you may sustain heavy losses, but I would guess you would fail a morale check once guys start dropping before you can do enough damage to make them take enough morale checks to rout (you can review these morale check rules too). I am not real certain on which troops C’tis starts with, though, and assuming they have a mix of their light and med infantry, and maybe even militia.
There are other things to consider, of course, like buffed morale (Sermon of Courage/location of battle), experience stars of units that add Att/Def/Morale, fatigue effects if longer battle, repel effects, but early on it should give you a decent idea barring doing actual battle testing in game.
I just consulted the manual and learned quite a bit more that should be added to this including how shields figure in, the chance they can get damaged or break (is this new?), the chance to hit different body parts causing checks vs helmets, and what the various damage types do. But, hopefully this gets you thinking on how to analyze this, and you can use the per unit logs to watch this in action.
EDIT: Hmm, it seems they now call it Harassed, the defense reduction when taking multiple attacks per round. It is also now only 1 def per extra attack (used to be 2?) and mounted units take half of that and it has a % reduction based on whether they are continuing to be attacked or not.