I'll write this for JMC again (as it's my client), but I doubt it's very different in Mudlet. It's very simple and not sophisticated at all, but lets you to quickly switch between preset 5 targets. Then you set your attacking hotkeys/aliases to {skill $pk} and you're set. There's rarely more than 5 available targets online, so this should be enough.
#alias pk #var pk - this just sets current pk target
#alias pk1 #var pk1 - presets target #1
#alias pk2 #var pk2 - presets target #2
#alias pk3 #var pk3 - presets target #3
#alias pk4 #var pk4 - presets target #4
#alias pk5 #var pk5 - presets target #5
#alias 1 {pk $pk1} - sets current pk target to preset target #1
#alias 2 {pk $pk2} - sets current pk target to preset target #2
#alias 3 {pk $pk3} - sets current pk target to preset target #3
#alias 4 {pk $pk4} - sets current pk target to preset target #4
#alias 5 {pk $pk5} - sets current pk target to preset target #5
#alias pkl {#var pk1;#var pk2;#var pk3;#var pk4;#var pk5} - shows you the list of preset targets in case you forgot them