Adding a quick page to give others an idea of strategy.
I always run a Recovery Necklace and a Stat Point Crystal.
I always keep def at 2.1x enemyDamage
I allocate agi to keep my combo rate at the 53% soft cap (which also means a 17% crit chance). This give an average damage multiple of about 2.4x with a 39% chance to do regular damage on any round. There is only a 2.5% chance to do normal damage 4 rounds in a row.
Next, I ensure my HP is at least 1.3x the enemy's base attack. This means regardless of my current attack, I could survive 4 hits from the enemy and still live (2.1x atk in def means dmg taken is reduced to 32%)
Finally, I need my Atk to be 11x the enemy attack to break even on life leech (accounting for a 2.4x multiplier), with <=2.5% chance to die on any given round. (note - Dmg is always 2.5x Hp)
These rules aren't a perfect allocation for any particular enemy, but they are a safe allocation at which you can expect to always win.
Enemies have widely varying Hp/Atk ratios, so here are some more specific numbers for *safe* hard mode farming. If you feel riskier, you could trade down 30% hp but add 40% more Atk, which is easier to reach thanks to weapon multiples.
Eye Boss - 100k HP, 250k Atk, 160k Def, 100k Agi
Shadow Samurai - 323k HP, 800k Atk, 530k Def, 100k Agi
(Work in progress - check back soon). I'm still evaluating whether "max combo rate" is actually a good strategy