I think wolf who build mass balistamen will eventually become unbeatable for serpent. With longer range than any of serpent unit have and astounishing atack power balistamen are able to kill canons and musks with incredible speed. Of course you always can make MORE healers to help your army last longer but it will seriously decrease your atack potential. Also, wolf can use zen counterpunch to quickly get rid of enemy heroes (including necromancer). It looks like this: upgraded druidess entangles hero, then blunt-and-pierce-resistant hero or unit (grayback, shale lord, gaihla, werewolf) with brawler behind his back moves towards serpent army to distract their gunners and runs back after job is done. Brawler will probably overlive his victim only for a second, but it doesnt matter. Honestly, being quite clumsy player, i cannot realize such combination personally, but im sure there are many players who can do it.
Its not even a question: in long game serpent's chance to defeat wolf is very low. Until balistaman towers stand, any attempt to charge on wolf's forces is doomed. You can launch a small suicide atack with group to of blastshots or raiders to destroy one or maybe two towers but all your demolition squad most likely will die. Its simply impossible to permanently smoke all wolf troops (especially without losing few to several canons) and even if you ll manage to do so, towers will still hurt your forwarding team badly. So, everything depends on your micro skills, speed and random luck. In any case you will lose some troops while taking down towers but you will still need to atack, cause its your only chance: serpent must act before wolf restore his fortifications. Distract remaining tower(s) with ronins/witches, drain them with witches, smoke, mug and blastshot wolf's army. Kill Gayhla quick or keep her busy in melee if you cant. Other Wolf heroes do not deserve your time: they rather damage absorbers than real threat. Use vetkin, taro and utara on biggest one of balista clusters. Keep blastshot and smoke balistas...
Still, even perfect performance of actions above cant guarantee victory for serpent. Anyways i cant imagine any better tactic
Some additional notes:
-Dont underestimate witches. Their combination of pierce resistance and blast damage makes them pretty effective against balistas. However, blast shots might do job better cause they damage few targets with one atack.
-Swordsmans with health upgrade can survive after glass-swording enemy. Use them in your defence positions to solve werewolf problem.
-Use some snipers to kill incoming zen counterpunches and protect your heroes. On the another hand, you can use mines instead. Thus, you will be able to equip more musks with blastshots, but this tactic is more risky for zen masters. About bandits' darts - these things arent good. Dodging skill makes them useless.
-Dont keep you heroes for too long near wolf's positions. No one can keep attention on a single unit for all time, so sooner or later your hero will be killed by brawler. If you need to distract balistas fire, quickly move your hero to lock them on him, do that you need to do with other units and command them all to go back. Use Vetkin instead of necro: he is faster and, after all, you can recover him more quickly if he perished.
-Dark pacting small groups of units going to complete dangerous tasks is doubtlessly good idea. Dont forget to heal them to full lifebar.
-Tamed wolfs cannot be commanded. So, you can try to lure them out and kill. Collect as many horses as you can. Every horse trapped in your stable or used by unit decreases maximal potential amount of wolfs avaiable to your enemy.
- Wolf troops power purely based on their brute force; this is not versatile clan (at least, compared to others). In a field fight serpent can kill Wolf fairly easy. Let them atack if they want to die so hard.
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I hope it will help.