Getting the Shadowlord on fell beast would GREATLY help your army. The shade is one of the best units in the game. Take it. Gulavhar should be good as long as he can get into combat with minimal injuries. If he gets injured before he can reach combat, use some magic from the SL to make sure he gets some kills.
Gulavhar should be deadly if used right (if you use him in conjunction with the SL then they can destroy everything). Use heroic fights to slingshot into heroes. (ie, if the hero is out of range, move into a combat that is closer, then heroic fight into the hero. I do this all the time with my dragon, it allows you to attack heroes over 18 inches away
, plus, your flying, so almost nothing can block your path)
I also recommend taking 1 bat swarm to follow Gulavhar (or the SL) around to help with heroes or to block people. you dont want Gulavhar/ SL getting swarmed.
long story short: the Shadowlord on fell beast is amazing, the shade is amazing, if u can use Gulavhar well then he is also amazing