Yep. I'm using a Dual Swords/Sniper Rifle build, so I cannot use Ghostwalker now. I'm spamming Blood Sacrifice, and Phantom Counter and Hellhound Skills (plus Crisis Catalyst and Red Zone for crits). The problem is that if I use it against normal enemies the fight is over in 10 seconds and against bosses, tyrants or stronger enemies I get killed.
The most fun I have is against large groups of weaker enemies.
I beat that boss yesterday with just one Skell. Me, Elma and Lin were level 27 and the fourth character was around level 22, so it's definitely doable.
Funny thing: my Skell has around 5200HP and Lin has around 5100HP. She's almost tankier than a giant robot.