OK, I could make it a full-round action, but I think - given the original description - that it should still be able to take its normal full attacks against other creatures (not the one being trampled) within melee range. Or that maybe it can only make its full attack if the opponent is helpless?
If we are moving into a creature's space, does the size of the victim still matter?
Here's a bit of revision:
"Piercing Trample" (Ex): As a full-round action, a hallucigenia can move into a (Large or smaller) creature's space within its threat range. It may also make a full attack action in the same round, but may only target creatures other than the one it is trampling. A hallucigenia's fourteen legs are very sharp, and deal 14d8 points of piercing damage to any creature it walks on. The opponent can make an attack of opportunity against the hallucigenia, but if it does so it is not entitled to a saving throw. If the opponent does not attempt an attack of opportunity it can make a DC X Reflex save to take half damage. The save DC is Strength-based. A trampling hallucigenia can only deal trampling damage once per round to a single target, no matter how many times its movement takes it over a target creature.