Death March is pretty easy, but wow taking a lvl24 guard on lvl6. Nice, nice I say again, that must of taken forever.
Anyways yeah the game only gets easier with levels, so if want a challenge you to restrict yourself. I restricted myself to no fast attacks, no signs, but then I just got really good at strong attack timings and not using signs got real boring real quick. So now I'm building a jack of all trades build, using some basic stuff in the weapon tree, Yrden in signs and focusing on potions and oils, but mostly oils. Really only skilled into the tree because I hate my oils running out.
I really think you could build anyway you want in this game and still beat Death March with little to no problem at all.