There's no best class. Like most slasher games, difficulty will depend on how you build and play whatever character you decide to use.
I've been playing for maybe five days now but I can tell you that if you want to be efficient, you might think about investing some fg into the game. There's a stickied guide to fresh characters somewhere. Some are more expensive than others.
In most cases, if you're killing slow it's probably because you haven't landed yourself a weapon with enough damage. Heavy weapons attack once per two seconds while everything else, I believe, attacks once per second.
However, in your case, I think it is your class. I don't know balls about the guardian class. From the reading I've done, I assume the guardian is a support character for heavy healing. So, I wouldn't try to solo with it.
Finding parties is just not easy to do. That's one of the reasons I'm interested in joining a guild with some LS enthusiasts in it.