I was messing around with the sentinel fight recently and found that if you activate the Spear of Achilles power as the hydra head is charging you it will pop the head. This means that 3 heads can be popped in the first cycle of the slow stage, which will let us reach the fast stage of the fight faster. Then there are only 3 heads left to pop only there isn't another cycle in the slow stage.

I've found myself getting soft-locked a couple times because of this but if you set an infernus that needs to blow up to open a door or gate, eg. Gates of Saturn or Gods of Battle, if you set them on fire with a fire arrow or the spear ability they don't actually destroy what they are intended to destroy. Not sure if this is just a GameCube thing though.