Just hit him twice, to stop HammerFall.
Basicly, never throw something out if you can't follow it up, or move out of the way.
If you throw a coin, then dash; get ready to jump if he does HammerFall.
If you throw out a K, get ready to hit S or 2S.
If HS, ready a MC or coin
If ensenga, ready an RC
etc
Your basic goto's against HammerFall are Coin and fast MC'ing.
oh, and OD beats HammerFall too.