TokenMod already (by default) affects selected tokens, so that part is already taken care of. Before getting to the specifics of the command line, however, we'd need to know which bar you use to link to the "health" attribute (you haven't told us what system you're playing, so I'm having to speak generally). TokenMod can only affect tokens; it has a small tendril of character alteration only through bars linked to character attributes... so if you don't have your "health" attribute linked to a bar, TokenMod won't be able to help you. For that, you'd need something like ChatSetAttr. Although, there is another script called SmartAoE which can create areas in shapes drawn on the map, detect which tokens are in that shape, and then apply damage. I know the author, David M., is nearly done with patching that script for Jumpgate, but if you're interested in the possibilities I'd encourage you to look up that script.
