Hit the shoulder. If he can't do that, then he shouldn't deliever the hit. Similar to a many other penalties.
You're allowed to check a guy in the shoulder. You are not allowed to cross-check a guy in the back. If you can hit a guy in the shoulder, hit him. If his back is turned and you can't hit anything but the numbers on his back, don't hit him. If you are unsure or incapable of controlling whether or not you'll be able to avoid hitting a guy in the back, then the onus is on the you the hitter not to deliver the hit (ease up on the contact, back off and play more positionally, etc.)....not to just throw the hit anyway and make the hittee pay the medical price for your wrong guess.
You're allowed to hit a guy in the torso. You are not allowed to hit a guy in his knee. If you can deliver a clean torso hit, hit him. If the only way you can make contact with the player is by sticking out your leg and making contact with his knee, you have to let him go. If the hitter is going to put his desire to hit a player over his responsibility to control whether or nor he hits him cleanly and legally, that's on the hitter.
You're allowed to skate in front of the crease. You are not allowed to skate into the goaltender. If you are incapable of or unsure of whether or not you will be able to skate to a point on the far side of the crease without running through the goaltender, then the onus is on the skater not to skate there or to take a different route that better avoids the goaltender. If you choose to skate there anyway and your lack of care/control/judgment causes you to make contact with the goaltender, you bear the responsibility for that.