Perhaps he realized, or maybe not, but in reality, the only reason Lux could resist against the Forgotten Beasts so much was because of his Divine Sword and the weird patterns that appeared on it.
Those patterns not only ensured that he could injure the beasts, but it also protected Lux from the nefarious time energy that could slowly kill a Primal Class after a while.
'Now it's just these four guys to take care of. The bigger threat is trying to recover, but I don't think it's possible for him to have enough time energy after that to continue the fight.'
The only reason Lux didn't focus on the other four was because he saw that they aren't as strong as Cartilagios, and this made him pay less attention to them than the latter.
Now that the most powerful beast was injured, Lux could take his time to kill the others and eventually end the fight.
