He was able to avoid the first attack, but he couldn't avoid the second swing that Loki did by rotating his body and bringing the claymore around for another swing when the first one missed.
The problem was that Simon couldn't see him very well because it was nighttime and it was dark everywhere. Only the snake could see Loki.
But Loki didn't seem to have the same problems that a human should have in the dark. Thanks to the Tracking Ability of Fenrir, he was always able to see Simon and keep track of his movements.
This was why Loki was able to make informed attacks on Simon's position. As for why his movement with the sword was so smooth and seamless, it was all thanks to the Sword Heart Metac.
Simon couldn't dodge the second attack, so the snake had to intervene to prevent him from being cut into two from head to toe. The snake blocked the sword with its tail, and as the claymore clashed with the tail, something unexpected happened.
