If all units were to compete Wimbledon-style...

[DGDobrev]

Well, there it is

Calculations complete. At the "ceteris paribus" rule (meaning "all other things being equal"), the Ancient Ent wins.
EDIT: Just a small clarification: Wasp Swarm damage is 50-90 physical + 50-90 poison, so I've applied the resistance to the first part of the formula in the xls file. Forgot to comment it for better clarity

However, with a pinch of luck, the archdemon can win. All it takes is that 1% better crit chance to work properly and give him a slight edge over the Ancient Ent.

I've used the standpoint that Archdemons must attack straight away, since the damage provided by Wasp Swarm at a distance greater than 4 squares actually equals the Ancient Ent's average attack damage. As such, both parties must start exchanging blows straight away or the other will gain an advantage (for example, if the Archdemon waits or the Ancient Ent defends).

Everything is in the xls table