For a Viking in Atlantica, the best choice really is to simply train up Freezing Axe as high as it can go.
If you do hold onto their Vikings beyond level 60 (Atlantica powerleveling and Atlantica gold), then it is a good idea to train up a bit on Guard Dispel or perhaps Break Down or Deadly Strike, but in the long run you really are going to find that your Viking is simply dying way too quickly to magic attacks, and you'll end up ditching him for another more powerful class C mercenary.
There are however a ton of winning PvP builds with Axemen as the main character in them. Their high magic defense totally removes the great weakness of the Viking class, and you find yourself with a main character with the prized skill Freezing Axe, through the full duration of your playtime in Atlantica Online. If your main is an Axeman, the obvious skill to train on him would be Freezing Axe.
From there, there are some other important considerations about the Axeman class' skills.
First of all, the skill Break Down is unique to each of the class D melee mercenaries. No other class has this skill. And the Viking is perhaps the only of the three class D melee mercenaries worth having as your main.
As regular mercs, none of the three are worth keeping around into the highest levels of AO, so having an Axeman main is a great opportunity to have the Break Down skill in the late game, and to combine it with Chaos Wind (a ranged merc skill) to produce the devastating spell Destructive Wind.
It is reasonable to keep a Gunner or Artilleryman into the later stages of the game, particularly in the case of stun formation builds for the Artilleryman. So if you have an Axeman main, it's a viable option to train up Break Down.
The other two skills, Guard Dispel and Deadly Strike, are also worth training for their own reasons.
Guard Dispel is highly powerful in PvP in Atlantica Online, as Holy Guard is indispensible in PvP, and Guard Dispel will cancel out Holy Guard on your enemy's units.
Deadly Strike, on your main character, could spell the difference between victory and defeat in close matches where most of the mercenaries on both sides of the fight are already dead.
In the long run though, training Guard Dispel is probably the surer bet, and having this spell into the higher levels is really a great asset to the strength of your team.
So, the best skills to train on an Axeman are probably Freezing Axe, Guard Dispel, and Break Down for stun builds with an Artilleryman in them. It is reasonable though to train up Deadly Strike alongside these skills.
