@Telethia There are some very neat ideas here. It amazes me just how versatile the BattleLore 2E system is for making custom units. Below are some of your abilities that stand out to me as being particularly original.
Rampant Growth: I never considered giving a unit a bonus for eliminating an enemy unit. It reminds me a lot of the Renowned mechanic in Magic: The Gathering. It's a neat way of putting emphasis on individual units. Unfortunately, I don't think your Huron will work very well. As a melee unit, you need to get adjacent to try to deal damage, and with only a single combat die, you'll have trouble eliminating a unit. If you try and fail, you're likely to get countered hard. How would you feel about a base of 2 combat dice and adding 2 when they gain a veteran token? That makes it more likely for them to take out a unit in the first place, but doesn't make them overpowered (after all, Berserkers and Reanimates can also get to 4 dice). Also, I struggle with the theme of a combat-value-1 unit suddenly gaining a combat value of 4 just because it took down an enemy. That's quite a jump.
Deadly Withdrawal: I love this idea! I never considered rolling a die as a reaction to another occurrence. It slightly adds to the complexity of the game, but I think for a fourth faction, it's an okay time to do this. When you really break it down, though, this is like a 1-die counter attack with the Ferocity ability the Obscenes have. One thing it does allow you to do is hit a Riverwatch Rider when retreating since this ability isn't considered a counter. Since Deadly Withdrawal is weaker than Ferocity, how does the rest of the unit compare? Getting a movement of 4 instead of 1 is quite a boon. Also, 3 dice is still pretty good compared to 4. The Agility ability also makes Leonx Riders more durable. They seem quite strong. When I think about them compared to other 6-cost units, I think they need to be taken down a notch. I think lowering their combat value to 2 would be something to consider. It's definitely weaker, but with their agility ability, they don't fear the counter attack as much and may actually get some damage out of it. And with a movement value of 4, being forced to retreat is not so detrimental, since they can quickly get back into the fight.
Again, I really like the thought behind those two abilities. Now I'll address the other units.
Elf Warden: He's too good, hands down. He moves like a common footsoldier, hits like an Obscene, and can act as a caster. I would consider dropping the combat value by 1 all around. So he would have a melee attack of 3 and a ranged attack of 2, but I would still keep the ranged attack to range 1-3. Actually, I have a question about Marksman. You said "at range 3." Does that mean he can only attack units 3 hexes away, or did you mean range 1-3? Having the versatility to choose between melee or ranged is definitely worth a drop in combat value.
Ent: It's very similar to a Chaos Lord. Basically, you're valuing +1 movement and the ability to immobilize as being equal to causing two retreats on a heroic [crown] result. It seems better than the Chaos Lord, but dropping either movement or combat value seems to put it beneath the Chaos Lord.
Elven Archer: I think a combat value of 3 is really good for a range 1-4 unit. When you add the ability to ignore blocking terrain, it makes these guys very powerful. I would suggest knocking the combat value to 2 and changing Sharpshooter to read something like, "This unit may ignore one hex of obstruction when determining line of sight. If there are no obstructions between you and the target, add one combat die." So if you take advantage of hiding behind things, you are less accurate, but can still take the shot. If you have a clear shot to the target, it is more deadly.
