Guild Wars 2

As I see on the Melandru runes it has 10%- condition duration/stun duration and then the sixth effect is 15%- condition and stun duration.
Does the 6th effect stack with the 2nd and 4th effect (meaning 25%- condition/stun duration)?

Yes, it stacks. Thats why this rune set is used by alot of warriors to negate conditions along with lemongrass poultry soup and dogged march.

It only negates Chill, Immobilize and Cripple. Damaging conditions still retain 45% of their modified duration. High Condition damage Necros and Engis with upwards of 90% duration can still put a lot of damaging conditions on you.

The main benefit is that with stability and 93% reduction to movement Conditions you can freely move about the field without being being slowed down. This is extremely beneficial as a melee.