Part of that is that I had a lone monster attack them. More would make life more difficult. Then, I just threw it in for fun. . . .
I will probably have to have them talk about how it's a good thing that such a wandering monster was so lacking in power. Especially since it caught them in open ground, which is not their strong point. (That's the thing about fighting in tunnels and rooms that are small enough to fit in dungeons. You build your tactics on it.)
May be wise to pile on the monsters more.