Tough to tell when we have yet to see Luslec's limits.
In terms of stats, Valhalla was said to have base power and speed close/comparable to a FH like Gustang who's using full Reinforcement. Luslec on the other hand could physically hold the upper hand against 15% Reinforcement Urek but was clearly losing against 25%. I don't think Gustang is Urek's physical peer or anything, but Valhalla seems more impressive here so far.
Valhalla's weak point is probably the lack of skills and versatility. She can shoot beams, attack with the spikes of her wings, and seems to have some sort of hypnotic power that came close to get Gustang. But that's it. Luslec has a wide range of spells with different applications, some of them quite complex (like the one that attacks infinitely until the opponent dies and spreads through the environment as spores). We have yet to see everything he has too. That's an important advantage.
Main issue is that Valhalla has infinite life as long as Traumerei is alive. Even if you tear her apart, she's coming back like nothing.
So, it mainly comes down to whether Luslec has a special technique that can seal/trap/immobilize/incapacitate Valhalla completely. If he does, he could potentially win after a difficult fight. If he doesn't, he's eventually getting killed.