Well +18 is not the highest enspell damage it's 23(if you use fencer's ring) for rdm, not counting hollow earring which is another 3 but I hardly see people get it.
If you count weather or day that's an additonal possible 1-3 damage at most. I can say that multiple weather has increased effect but not sure how much precisely.
Sch can do the aoe version of rdm spells now if they sub rdm, it was out well before the "frontline update"(?).
According to wiki enspells can achieve a 20 damage base, so add fencer's(5), hollow, (3), and weather(1-3) you should be able to get up to 31 on a lucky blow.
http://wiki.ffxiclopedia.org/wiki/Enspell
http://wiki.ffxiclopedia.org/wiki/Red_Mage:_En-Spell_Damage_Calculations