Like Xaven said, keep changing characters every stage- that's what I do too. I have four characters and i've gone into my 17th year just for fun and they're still not the best they can get! It takes tons of time- there isn't a shortcut. If you finish a level quickly it'll only reward you with not so good artifacts (+1/2).

Keep doing places that don't need myrrh if you don't want it to get harder. Getting myrrh make it harder. Just repeat dungeons with out myrrh...

HOWEVER

Certain level have some artifacts that can only be obtained in that level so you might have to do places with myrrh and surely enough, it's not going to be quick. Looking out for the Ultima artifacts which give you +10 would give you a boost but you have to go into the hard bits of the game just to get it.

Hope that helped!!