This is a question concerning Gimli's action in battle in The Two Towers. At Amon Hen, Legolas sais "we (he & Gimli) slew many", but later, Gimli sais to Théoden that his axe has seen nothing but wood since Moria - which took place before Amon Hen.
just a thought that maybe he was referring to the time around about when they were in Moria. Both evens (Moria and Am on Hen) happened fairly close to each other...
upon reading the book for what probably is the 100th time, the book said Lego las and Gilli were hunting Orcs in the woods and ,as you said, "slew many"... "Hunting" probably says they were sneaking up on and killing, or cs, not actually having a full on battle with them so Gilli's battle axes wouldn't have got a work-out that time, just his little ones.