Derby is a one-trick pony. All he has is his feet. Havey's spider nature makes him suited to all-terrain far moreso than Derby. It's as simple as him climbing up and having his way with the unprotected top of the mech.
<Snipped quote by Cpt Toellner>
I don't give a fuck if you say this, but one thing I find kinda funny is: I've seen you and others say this around here, but the one time I said it, Sherlock deleted it for offensive language. Lil bit ironic. lol