When I got onto that Steppe, first monster killed me. Tried to fight it again. Killed me. Then went on to next monster. Beat it. Boosted with confidence, I attacked one of the giant turtles. It stood on my head and killed me.

Basically, you're not meant to be able to kill them. What I did was followed the first couple of Marks (or C'ieth quests, whatever they're called) around the Steppe, then the ones that went the way you come down from at the beginning, then moved on with the game.