Since you're main dmg will be coming from spamming q build, the fel flame quest at 1 is essential. More mana from life tap gives you unbelievable sustain when in team fights at lvl 4. At lvl 7, grab the corruption talent to slow enemy movement speed in order to help your team secure those kills, landing your E and following up with Q gives you easy kills. Haunt at 10 is the one and only ult for Gul'dan, make sure that you time when you use it very carefully. The enemy will be "haunted" to continue running in whichever direction they were facing when the haunt was popped. You can definitely use it on one person who is isolated to secure a kill, otherwise save it for escapes or to keep them off you for a few seconds so you can get back to your team if your split, tap fountain, and re-engage the fight. At 13, the Q spell armor is essential against comps you're facing with mages, spell dmg heavy teams. The only other alternative at 13 is DARK BARGAIN which is extremely strong vs enemy teams taking a zeratul/stealth, or divey heroes such as kerrigan/al'arak. That extra health will keep you alive. At 16, you want the Q talent again since the extra 40% stacked dmg when you hit 5 heroes is reached your damage is just unreal. Haunt at 20 gives you easy picks, lowers enemies armor, and increases kill potential.