There's probably lower hanging fruit than this, but if you are aiming for the lowest RAM use possible, don't require compatibility with third party binaries, and don't expect to exceed 4GB, running an i686 build will reduce binary size and memory consumption across the board. There is an unofficial 32 bit version of ArchLinux available (https://archlinux32.org/). Never tried it myself though.

Compiling a custom kernel and vastly limiting the number of enabled features can make an impact. If 250MB is too high, even the choice of filesystem (and number of filesystems mounted) might have considerable implications on memory usage.

The main sticking point is that you are stuck with the binaries Arch ships. Perhaps you can replace some with leaner alternatives versions provided in AUR, but the key to reducing RAM use below where you're at now is going to be completely disabling optional compile-time features.