Humphrey attacks with a suitably large hammer and has a variety of attacks. His attacks include a simple hammer swing hitting directly in front of Humphrey's orientation, a hammer swing that sends out three small shockwaves in a fan shape from the area of impact, a 'short' jump towards the player that sends out a large circular shockwave with limited range, and a 'high' jump where Humphrey jumps up out of view, coming down on the player (or where the player used to be if you're moving) and sending out eight small shockwaves with extreme range in both cardinal and intermediate directions. Humphrey's 'tells', or indicators that he will use a specific attack, include a distinct charging noise to indicate a hammer attack with shockwaves (as opposed to a hammer attack without shockwaves), and flashing white to indicate a jump attack.
When Humphrey lifts his hammer for either of his hammer attacks, the key to not getting hit is to not be in front of Humphrey. Simply moving to Humphrey's side or back will avoid the attack, and give the player a moment of leeway to attack him without retaliation. When Humphrey begins either of his jump attacks, start running immediately. If the jump is 'short', keep running away as long as possible until he lands. If the jump is 'high', he will land with his eight-directional shockwaves. The best way to avoid this is to be moving straight in one direction until right before he lands, and then switching direction by ninety degrees for a moment. Performing this correctly should put the player in one of the eight shockwave-free zones. Attack him while he is between attacks or facing another direction, and then get out of the way.