If you want a challenge, see how *fast* you can do a run.
The best I've got on the current version is a 6 turn run (5 turns used to be possible, not sure if it is any more)
It's pretty hard!
I've got to the stage of "kind of Consistent 7/8", but getting below 7 is *really* hard.
Generally speaking:
You want to do 4 or 5 damage on turn one (2 skelletons, 2 skulls is best opening). and 8 or 9 damage on turn 2, 11-12 castle damage turn 3.
Skulls and bones are the way. (Two skulls and one bone is three purchases and gives 5 damage to castle). Teir 3 Summon book also pretty critical if you are trying to do a rush.
The other critical thing: If you are trying to race, Life is a resource. A resource to be spent. When in full rush mode, I often end up leaving an entire row empty in round 5 or 6, meaning I take 5 or 7 damage that round... but pushing through 20 damage in exchange. By the time I START the king fight I'm often on only 1 or 2 hp. So long as the king dies before I do, its a win.
... With that said, leaning heavily on the "rush only" strategy can be less fun (you just ignore a whole bunch of fun and cool units because they are not part of the "max damage every round" game plan.