Currently, there were only 200 metres left until the pitfall. If Sheyan seized that opportunity when the troll was uprooting to sprint fully, undoubtedly it wouldn't be difficult to flee to the pitfall. Instead to Sheyan, this wasn't part of his plan and ruin the bigger picture for escaping from the range of attack by this mountain troll. If he didn't take a strike from this troll, primarily he wouldn't be able to obtain first hand information of the mountain troll. This meant he would not be able to accurately access the fighting strength of the troll. Apart from that and more importantly, if he ran back in perfect state, then no doubt it would attract the unwanted attention of the Symbiosis sect and Diaz. That outcome is one that Sheyan definitely wanted to avoid.
Therefore, he had to face one strike of the troll head on. That was the only solution to his problems!