You only need to do that if you want to hit the shrine and activate the run speed buff.
I personally only care about modorboat, and never use the others, but if you swap them around or like this buff you will need to kill it again. The good news is that he is like 2 hits at higher levels, so you can do it later. As long as you have his antlers in hand, you can put it aside forever or for later. Ive killed him a couple of times just for a head to hang up at my house.

The worlds are HUGE. It is unlikely your world is too hilly everywhere.... you have probably explored like 0.01% of it. You can also flatten the ground yourself, but its a dark art and takes some time and effort. Big flat areas exist where there are several falling apart buildings together, but finding one of those in a good spot can be tricky.

you can also swap worlds at will, so you can leave the old world and return to it if you want.