I had 0 problems with my non-Ult SLKR, also with GAS+501st i could do it without much trouble. I think it might be a disc issue? What discs are you using and what others have you available to use?
If it helps, these are the discs I'm using:
- Weak Point (Green) --> A must
- Critical Debuffs (G) --> The other important along with WP
- Defensive Formations III (B)
- Caustic Emissions (G)
- Entrenched (B)
- Leader's Resolve (G)
- Debuff dismantler (Kinda useless in my set up)