5e doesn't give a lot of rockets to use though. No class can recover 6th level slots, and a build with Action Surge will never get the 2nd 6th level slot.
However a bard using magical secrets at high level can pick up Disintegrate, Divine Word, and Power Word Kill. Divine Word can't be used in the same turn as Polymorph, but Disintegrate and Power Word Kill can be used via Action Surge once per short rest. The bard can end up with 3 rockets and Action Surge or 4 rockets at 19th level with no Action Surge.
Arcana cleric can pick up the rockets too but someone else has to cast Polymorph. Theurgy wizard could grab all the spells too.
I suppose I should step back though and point out 13th level or higher characters are beyond the levels most people play, and we already knew high level spell casting is ridiculous.
And one more edit: Bard wins, because it can also get Conjure Woodland Beings. 8 pre-summoned pixies polymorph stuff. Each successful poly is an auto-kill via Divine Word. The pixie DC is low, but a creature might have to make the WIS save up to 8 times.