Among the methods that Sheyan could think of, the simplest was to burrow into the ground and cover himself up with soil like a mouse; quick and convenient. Although these poisonous insects were huge and ferocious, there was no way they could dig.
However, unless Sheyan had the ability to make tunnels and travel underground, he would only be digging his own grave, because once Treebeard arrived, all it would take was one stomp to kill Sheyan.
Sheyan's only AoE attack was to throw a curse-mix. It would produce a shockwave that would damage and stun the surrounding enemies. The damage would be pretty significant too.
The problem is, the cooldown of this ability was thirty seconds. Not to mention that if he wanted to make a curse-mix now, Sheyan would have to exchange for materials from the nightmare realm. The price would be astronomical; each bottle of curse-mix would cost over 7000 utility points.