Evading enemy attacks is key to preserving health and keeping your combos alive. Get used to hitting the button between attacks.
From the options menu it's possible to reconfigure the placement of the analogue stick and virtual buttons. Unfortunately, the buttons can't be individually arranged, only as a group.
Make sure to press "Confirm" after selecting an upgrade. If you forget to do so, the health or combo selected won't unlock.
Rack up extra points easily by breaking barrels situated throughout each level. Sometimes they're slightly hidden by the camera, so search thoroughly.