i must say i do suck at singleplayer personally and im not really expierienced with it.
what i´ve gathered from threads is that when you really do some damage to uke you should have an speed between 35-40 iirc.
you can see your speed by typing /sm 1 ( or /sm 2 i dont know the diffrence they both show the same thing everytime i checked ).
to gain momentum there are quite a few techniques.
but it usually has to do with beeing relaxed then turn your tori ( work with your chest , pecs , lumbar and glutes sometimes ) to achieve higher speed / more momentum.
also for singleplayer it does help if you play it with 1 turnframes instead of 10.
for beginners im pretty sure it would not hurt if you turn the dismemberment threshhold down to 80. i would not go lower than that because you will mostly damage yourself.
like i said i dont really play singleplayer so i might be wrong about the settings.
im pretty sure you can get good answers from the ORMO org.
http://forum.toribash.com/forumdisplay.php?f=273
they even have a thread about questions like yours and im sure they´d be glad to help you further improve your gameplay.