I recently used a Pacific Rim sized samurai mech in my OA campaign. I used a slightly modified version of the the Usurper. Because I have 7 to 8 PCs I allowed each section of the mech to be piloted and that section took an action on the PCs initiative count. All extra PCs sat in the heart/cockpit. I did reduce the charges per rod from 20 to 10 to accomodate the number of PCs I have.
I then had it set upon by a “Kaiju” (tarrasque), Oni Bat King (evil Phoenix, or, “the bat out of Hell) and the Oni Wind King (elder tempest). A Void Yai Oni (the fine Big Bad for my campaign – Anumaramon) was also harassing them and he was the one opening gates and brining the kaiju to attack the mech until he was driven off by the PCs stopping the flood of kaiju’s. The mech was able to take them all out with 24 charges left (out of 70) and pretty much at full hit points. Was fun and very different.