GENERAL

• LEARN HOW TO PARRY! it's the first thing you should learn while on your way to win fights on higher difficulties. to parry, hold directional keys into the opponent (so forwards if your camera is facing the enemy) and press light attack. this will make you hold up your block for ~1 second (half a second on very hard and revengeance). to block again, you need to release and press both the directional keys and light attack! there are few attacks that cannot be blocked, they may be telegraphed by a yellow flash from enemies (e.g. Armstrong's grabs).
• learning how to block will lead you into hitting perfect parries. if you put up a block fast enough before an attack that can be perfect parried (about half of block interval - 1/2s below very hard, 1/4s otherwise), raiden will deal a powerful counterblow. this will stun most common enemies and occasionally deal monstrous damage to bosses. on revengeance, although most bosses hop away from perfect parries, Sam and Mistral can be perfect parried for more than 60% of their health with the FOX blade! this is an essential tech for ""cheesing"" fights.
• defensive offence ( Ⓧ + Ⓐ ) makes raiden strike and hop away backwards by default (you can adjust your direction with directional keys) and gives him invincibility frames (which do NOT last for the entirety of the move). this move is crucial to revengeance boss fights - the only way to defend against unparriable attacks is to dodge them, since most will deal near-fatal hits, ESPECIALLY armstrong.