If one of my units is webbed and the other is one space away, why would the spider move to be in range of both units' melee attacks?  It is to the spider's benefit to take on one enemy at a time.  

A very crafty spider would web one and beguile the other and then move to put both enemy units adjacent to each other.  

Also in nature, a venomous creature generally strikes and withdraws for a time (kites !?!?) until it's prey is dead.  A 3 or 4 move spider/naja/plague beast could do this quite nastily against early champs and troops.

Just some thoughts to improve the enemy AI.

Have you considered randomly envenoming the weapons of bandit units, ruffian archers, troll warriors etc. to spice things up a bit?  Say 10-20% of all encounters might have one or two units capable of poisoning my troops might be a fun challenge.  Or an extra bonus for a Bandit Lord sov...