Some Tactics:

Amakusa in SS3 is one of the weakest characters (That's very sad). But this does not mean that he is hopeless. In fact,

1) Amakusa's range is not very long in SS3. And his normal slashes isn't very helpful either. So mainly rely on his crouching kick. The combo (d+D)x4 will make your opponent dizzy, that's the most important combo of Amakusa in SS3. However, after your opponent gets dizzy, doing this down kick 4 times will not redizzy your opponent. (d+D)x4 will only make your opponent dizzy if he is in non-dizzy state.

2) Amakusa's 421+A (i.e. bdb+A) does not have damage but it makes your opponent dizzy. Use it as antiair, or in close range as a suprise attack. However do not overuse this because it has long recovering time. The good point is that it comes out very fast, so can be used as a suprise attack in close range.

3) Amakusa's strongest slash is his far crouching C (strong slash). Yes, not standing or jumping C, and not crouching near C. So after your opponent got dizzy use this. Be sure not to be too near your opponent or it becomes a crouching near C with much less damage. Also not too far from your opponent or you cannot reach him.

4) Slash Amakusa has a better projectile with less recovery time. Bust Amakusa has a better supper because it comes out faster, so it can be comboed after a standing close mid slash. Someone says that Bust Amakusa's 646+slash (fbf+Slash) is better than Slash Amakusa's, because the former is in ground portal form which comes out faster. I don't quite agree on this since slash Amakusa's 646+slash is in the form of "Ball in mid-air", which can restrict the opponent from running towards you.  Both have the slapping move in SS4. Which is better? Probably Bust, since he has a faster super.

These are from my personal experience... Please feel free to contact me if you disagree.