Got it.

The master key, ie "1" needs to have this macro

/cast Elemental Mastery
/cast Nature's Swiftness
/cast Chain Lightning

And the slave key needs this.

/cast Elemental Mastery
/cast Nature's Swiftness
/assist [mod:ctrl,mod:shift,nomod:alt]Eeny;[mod:ctrl,mod:alt,nomod:shift]Meenyy;[mod:alt,mod:shift,nomod:ctrl]Miiny;[mod:ctrl,nomod:alt,nomod:shift]Moeh;
/cast [harm] Chain Lightning