Jaws: Yeah I like decoy. That sounds pretty good.
Yata's Reverse: First of all, the whole "end phase discard" thing looked weird, since it wasn't really specifying whether the person has to ENTER battle phase (in which they would have to discard every turn regardless) or otherwise. Also, the second ability has nothing to do with the first ability, and doesn't specify whether this ability applies to JUST you or all players (as the card seems to imply). That's all the bone/beef I have with that one.
Serpent: There are 4 abilities in that single monster, with one of them being plain dumb (the attacking reptiles one) for being too specific. The other two abilities (the last 2) are alright (mind you, the whole "destroyed" thing doesn't specify when the ability goes on the chain), but they stick out like a sore thumb when combined together. My suggestion here is to split this into two cards (I guess both with the "can't be target" thing) and it'll be very good.
Nile god: Looks alright to me. But it might be a little too much on pushing the borders in terms of brokenness.
Ballista: Yeah yeah it's alright.