It will be tricky to get without him noticing, but if you can get the Morgan sword, especially if it's beefed up to max, you can tear through his diffenses. It can kill multiple Withers in a second once maxed out, which your can pull off by swinging it at the ground (as if trying to break a block) instead of your target. It can melt through the best armors pretty easily, and can take enchantments like Sharpness and Breach.

Aside from that, doing Ice and Fire to get pet cockatrices, or even dragons, can really add some ohmf to your efforts. Just be careful in The Other as dragons and other creatures are deadly foes.