Spawn as many 'summons' as possible as the number of Prismatic Gem beams increases with each spawn e.g. Companion, Tenno specter (eg [Octavia] holding proboscisc [Cernos]), Syndicate specter (ancient specter is most useful), Clem Clone, Kahl air support, on call crew gives about 6 beams.

Weapons
Use a heat inherit primer to allow Prismatic Gem generated heat procs to inherit the heat mods and one faction mod from the primer:

Bubonico alt fire heat primer - https://overframe.gg/build/440064/
Cedo alt fire heat primer -

Companion
Nautilus for passive grouping: https://overframe.gg/build/540870/nautilus/hydroid-arsenal-the-whirlpool-nautilus-grouping-priming-dps/
Can use Verglas for even more semi AFK DPS: https://overframe.gg/build/234877/verglas/afk-steel-path-melter-companion-dps-heat-viral-verglas/
Can use Heat inherit Helstrum primer : https://overframe.gg/build/547236/helstrum/helstrum-primer-heat-inherit/

Archon Shards
Emerald Shards - 2 emerald shards with +max corrosive stacks can allow Prismatic Gem to apply corrosive stacks (via [Archon Continuity]), which allows it to full strip and prime enemies.