Assault Crash is not really needed. It can be useful if you have some MP leech on your Dagger or maybe one HP (Rohan Gold) leech on your sword. You use Order Swing to charge up Assault Crash. It's good for soloing in areas that have mobs dealing a heavy amount of damage, however, it is far more productive to just get a damage drop weapon, a shield, and tank the lil fucks and rape them.
So if you think you must have them, and then get them. Level1 (Rohan Crone) only. Assault Crash does a little less than your normal hits. Like 10%-20% less. Most people do not expect a Guardian to smack them with a charged up Bleed. Bleed is great for bosses and as one of the finisher moves when you use Order Swing in PvP. If you are fighting someone who does, then hit Bleed near the beginning of Order Swing. Make them waste Detox Potion, and then smack them again at the end of it. Have a good time in Rohan.