I understand your frustration. When I PvP'd at low levels I too experienced being one hit killed by a treasure card attack. To combat this I started carrying treasure card weakness and tower shields, as well as a few mighty attacks of my own in my sideboard.
Another option would be to level up your wizard more, but the bottom line is high level PvP players use their sideboard to their advantage.
And these days, with the length of Promethean matches potentially being so long, I've noticed a lot of players must have started another wizard and are PvPing at much lower levels, so lower levels are probably saturated with warlord level players right now.