A preliminary observation: if among your minion you do not have a tank (or at least a ToJ), an Axbow is utterly useless: an exbow can be useful in more situations.
Since an Axbow targets tanks, you only have two options: either the axbow-wielding minion has enough DX to hit the enemy tank (and this means that your minion will have to be also a tank), or you rely only on the + to hit. I do not think it would be a good idea to train DX on a mage/enchanter only in order to be able to hit with the Axbow. Therefore, my proposal is to rely on the PtH of the Axbow.
The setting I had in mind is: either EETM, or EEET, and the Axbow either on the mage, or on the 3rd enchanter. The enchanters train AS, VA and your favorite EO.
Note that, unless something exceptional happens, your minion will survive at least up to the 3rd round of ranged combat, and usually will die before the second round of melee (or it is a mage, and will not use the axbow in melee), hence your axbow will fire twice.
BTW, another possible setting is EEWT, where W is a wall, with the Axbow. However, wall means CoBF, which in turns means that tanks will not be a problem anyhow.
The question is: how much should you upgrade the Axbow?
I would say: take the + at about +50, and the X at as much as you need: on average, your axbow will hit once per fight.
How much does it cost? Let us see at the most valuable items stat:
An Assassin's Crossbow [4x44] (+2) worth $4,539,647 owned by Melris (The Blacksmith)
An Assassin's Crossbow [4x33] (+54) worth $4,404,563 owned by Rufiolo (Hogwarts)
An Assassin's Crossbow [4x38] (+45) worth $4,263,625 owned by Nonsensical Knight (Neji)
NK's Axbow seems to be pretty much what we we want: it costs about 4mil in the blacksmith (significantly less if you forge it or have it forged). It will hit more or less once per battle, draining nearly 10% of the enemy tank DX. A 10% DX can be quite important: see A Dex based hits.The interesting fact, IMO, is that the alternative is simply no ranged weapon at all on the enchanter/mage. The downside of this approach is double: the cost of forging the axbow, and, most of all, the new proposed malus you get on your rewards from having high NW. However, since the malus is not operative yet, I do not know whether you should take it into account in this contest...