Maplestory "High Five" Private Showcase

How did he get so high total dmg? Also 77% final dmg ? Does that mean final dmg is addition ? If I had 30% & 40% final dmg skill , what would be my new final dmg be?

I don't know, and no, why having 77% final damage mean that it's additive? If you had 30% and 40% final damage, your window would show 82% final damage (meaning you do 1.82x damage, since 1.3 * 1.4 = 1.82). Maxed Cross Over Chains (+80% Final Damage) and Level 10 Final Cut (+20% Final Damage) adds up to 116% final damage in the window (1.8 * 1.2 = 2.16x).