So after the mission Pierce of Cake, when getting armour/helmet/shield suited to defense against piercing attacks, what is the best armour to get that is availible at that time?
What is the best armour to get?
To expand on what Aureate said: Advanced players in this game tend to favor armors that give offensive bonuses. So, depending on whether you use swords, guns, or bombs (or a mix), you should start your armor process by reading the Swordmaster Guide, Gunslinger Guide, or Bombing Guide on the wiki. Consider also my own sword guide and my list of 5-star items.
Unfortunately, bombers don't have any piercing armor. So, in the end, it comes down to Skolver, Justifier, and a few others (Deadly Virulisk, Mercurial Mail, etc.) that are generally considered less good, although they may offer slightly better defense.
I consider Mercurial to be the bomber piercing armour. The bonus is not directly related to bombs, but bombers are the ones getting the most benefit from it.
.....actually, I find that gunners using argent peacemaker/alchemers get just as much, if not more, benefit from MSI than do bombers. But thats just me.
I would personally say either Wolver or Gunslinger, depending on whether or not you like guns or swords. The former is available at 2*, and is reasonably cheaply available in the AH. Gunslinger, unfortunately, is 3*, and will be about 15k cr in the AH if you buy it from there.
The skint option would be Cobalt, which is free in one of the reward missions. In the long run, though, it sucks.