How is the score on the leaderboard calculated

If I recall correctly the main dungeon has 35 floors. Of course, you may play an infinite number of floors even before finishing the main story, on account of treasure maps and the like.

I can't say for sure what is the 'best' build, but one thing I've found that made the game much easier when I played it is having a very high chance to reflect projectiles. I built my vanquisher mostly like you, focusing a lot on ricochet and exploding shot, ranged damage, and critical hit and putting most points into dex, with some into other stats as needed for better gear. Basically I was a glass cannon, the only problem I had was survivability from ranged and magic attacks.

One thing that made my game exceedingly easy was to find a pretty low leveled shield with +20% or higher chance to reflect projectiles, and then enchanting it a few times. You need to get very lucky (and let the enchanting succeed 8-10 times without wiping the item). On top of that, what you want is to have the item to go over 100% chance to reflect ranged attacks. This, in effect, made you immune to all ranged attacks as well as magic (I'm not sure if this is currently still the case as I haven't played the game for a year). This made the Dark Zealots (and their insane damage magic attack) much easier to handle. All I had to do after getting that shield was to maintain my distance and kill everything with exploding shot.