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It's just a matter of keeping the run going long enough.  The longer the run goes on, the more the kill total scales up and the more Golden Whetstones you can find.  My current run is on floor 190, I've applied 11 golden whetstones to my dagger, gotten nearly 1400 kills with it, and my first attack of the fight is over 5k damage (it goes up from there because of +rage/attack from the forge).  Basically, if you don't get killed you reach a point where your damage output scales up faster than enemy hp does (the Shield Spirit + Hercule Pavise combo is pretty important for this because it turns it from 2 energy per attack to multiple attacks per energy - if I cleared out the space to activate my Red Energy I could do 72 attacks per turn right now - and it gives you free block at the same time).

2 and 3 are both definitely problems, and for the same reason; it's important that when the same language is used for different effects they work the same way.  The two effects on the Shield Spirit both use the same language to describe when they activate, therefore they need to both activate under the same conditions.  If the player had an item with the same text as the Elder Mage's ability, it would cause them to dodge every second damage source in a turn, not all but the first; therefore the Elder Mage's ability should work that way too.  Consistent use of language in effect descriptions is vital to a game like this.

The issue with vampirism counting as a separate damage source isn't technically a bug, it just causes problematic interactions due to ordering (and changing the order in which the damage sources trigger doesn't solve it, it just changes which specific situations have problematic interactions).  If it were me, I would change vampirism X from "deals X damage and heals for unblocked damage dealt" to "this item heals for unblocked damage dealt up to a maximum of X" and give most or all of the items with vampirism X an additional +X damage bonus (that would keep the total damage and potential healing the same, but unifies the damage into a single damage source - technically a small buff, but much more consistent in its behavior).

Just passed floor 150, no signs of stopping.

I do have some bug reports, though.
First, the Assassin Dagger's kill counter starts behaving oddly once you get it high enough.  Past 1000 kills, it starts counting by 10s (the damage bonus still only goes up by 1, though).  For example, at 1121 kills it will say you have 2210 kills and a damage bonus of 1121.
Second, there seems to be a bug with Hercule Pavise activating multiple shields at a time.  Some of the activations will be double-counted for negative effects (enemies gaining rage on shield use, damage penalty from Shield Spirit) but positive effects (attacks triggered by Shield Spirit, gaining block, etc.) only happen once per shield.  In the setup shown above I get 10 shield activations, but 17 triggers of negative effects.
Third, although on-damage effects were fixed to only trigger upon taking damage, the fix only applies to the player - enemies with on-damage effects still trigger the effects if they take no damage from an attack.  In particular, the Elder Mage's +1 dodge when taking damage still triggers when avoiding damage by dodging, meaning that only the first attack of each turn can do damage (and what's worse, if you have a weapon with a small amount of vampirism attached to it, the vampirism will trigger the dodge effect, preventing the weapon from ever dealing its normal damage).