Your characters advantages, I might add, are still surmountable. Complete knowledge of the local area, now manifesting in total invisibility. This was actually the only point where Higan himself may have had an advantage, seeing as your character has an inherent ability to become immune to everything once its used once, his first strike was surely going to be his best.

The trees limit both visibility and manoeuvrability for the Mech. Using its primary weapon would be highly problematic and unlikely, it also has to deal with numerous plains of engagement, above, forward and behind. The Mech doesn't really have quick or lethal melee capabilities, so at close range the Pater can test its durability and fuck around with it with quick darting attacks. The Pater could also just throw rocks at it or some other similarly annoying action. 

Also Higan could run out of ammunition, and ultimately the Pater can disengage at any time and there's really nothing he could do about it, so it's unlikely the Pater would even lose/die if Higan gained a strong advantage over it. Anyway, if you're going to fix the echo-location disparity we can stop OOC'ing and move on.