How do you get to wave 15 of the pumpkin moon in terraria?
Maybe this is a silly answer but I found that it was trivial to get to Wave 15 with one other player fighting with me in a simple arena made of a dirt pit with a couple levels of platforms. We both had level-appropriate equipment (we only used our characters while playing on our specific world together) and could effectively kill anything within seconds of it spawning, going through multiple waves every minute.
Everything's easier with a friend!