I think that it's completely deadweight for Mage decks, and, to be honest, for every other deck as well.
Let's compare Archmage with other rare Mages.
Elven warlock - VERY heavy attack, basically costs 18R when your Magic is 6 or higher, can give a load of gems if played on Alliance token.
Lord of time - Swift/Rebirth, cannot raise Magic, but returns 12G every time when played, also has pretty decent attack/cost ratio for a card with this keyword combination.
Luna the Sorceress - Far sight, +1 Magic, decent attack.
Master of the past - very, very situational, but also very strong.
Myth summoner - relatively cheap, +1 Magic for Mage deck, summons random but definitely strong card.
Thor - Swift, really good attack by itself, -5 stock when comboed with Chain lightning, potential for Nature chain start.
Wind witch - Swift, some attack, rare Aria, targeted discard, free with Magic 8.
And, finally, Archmage. Pretty low attack, -7 enemy stock, summoning last card. It's not even remotely effective as attack card - 20 attack for 25G. Uncommons like Poseidon's wrath and Firestorm are better than Archmage.
What about utility part? Actually, the same story. That kind of utility exists on uncommon level, it's called Rejuvenation, it's much more effective and three times cheaper. Not to mention that such kind of utility makes your hand stagnant.
Yes, there is a possibility to chain it with Stonehenge, but if you play Mage deck, you want to use Stonehenge ASAP, and in 95% cases you won't be able to play Archmage after it. Or it won't be that big of a deal, if you decide to keep it. And yes, it costs A LOT.
Overall, Archmage seems really, really weak, especially for a rare. This card has zero synergy with Mage deck, very prohibitive cost, low attack and low utility, so, I guess, using Veteran militia on it is the best use of it.
I'm not sure what to do with Archmage to fix it. Here's some ideas:
1) replacing a card in hand with last card played;
2) adding Swift;
3) adding another effect, preferrably defensive or resource returning;
4) lowering its cost to 13-14, attack to 15, and stock reduction to 5;
5) summoning/replacing a card in hand with Ice queen.
I like the idea with Ice queen, but I don't know what Mojko thinks about Archmage's place in game as a whole and in Mage cardset in particular.
Any thoughts, opinions, ideas and critique would be appreciated.