I doubt its an exploit, but it is not very fair. I think the spirit armor is overpowered, and should not be able to be used in PVP. It just makes battle too hard. As am myth student, though, I find minions very useful, but they are a bit unfair. Reshuffle, however, is also overused pvp. :?
Wow, I even use a few treasure reshuffles on my level 10, it's just part of the game. Any good upper level player uses reshuffle, especially on the small decks many run. I see nothing wrong with Reshuffle, as without it you cannot multiply the cards that you need, in these long battles. So, for me, reshuffle is absolutely needed in the game, and at almost any level.
Most people forget that reshuffle was designed to reshuffle discarded cards back into your deck not used cards. I do not understand why developers decided to change reshuffle to what it is today (reshuffle all cards). I personally like the original game developers. The newer team just is not in touch with the community.