Reply 1# Ace Of Spades
Merc's suvivability gets better in T4. Zero damage taken by stacking skill damage reduction and damage reduction buffs is back, making the survivability build more complete and back to the old days before T2. However, Devastating Howl's potency is lost. You can only get 70% attack reduction on monsters by stacking Devastating Howl and Taunting Howl.
Regarding not maxing whirlwind, the reason is due to the long cool down and casting time. In addition, Whirlwind no longer do 36 hits on big mobs. So, I prefer maxing circle swing which I use more frequently than whirlwind and aslo to prepare for the upcoming EX skill. If there is ever a whirldwind EX, then I would switch from maxing Rolling Attack to maxing Whirlwind
There's another bad choice of skill that i have made, that is regarding Rising Slash. Cricle break might be better for PVE because you can easily deal the full damage with one hit and it is faster. I kept Rising Slash because I still do some PvP and you need it for combos.