Your levels are more than good enough. I won with my three main characters at level 27. Other than that, follow everyone else's advice. Equip Nishijin belts if you have them (I didn't and I was okay), spam thundara and try to nail a quickening chain when Mateus is down to a quarter/third of its health.

As for teleport stones, my advice is to hunt marks. They're often given as rewards, and also they'll be avaliable to buy in the clan shop in the bazaar once you go up a rank or two.