I don't know the exact number of time bombs you need, but wwith the ammo packs of the pilot you got enough and you can do it much faster then two minutes. You put one on the neck, back of to let it explode, and another one at the neck etc.
When the pilot hands out ammo packs, be sure to stand near to eachother. The ammo packs will fall righ trough the AT-AT if you don't 'catch' them.
I have to add we didn't try this on a moving AT-AT, that might make things complicated...