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.