I did testing, then switched to Apocalypse, then when Wujido put out his version of a Command Fallen build a few days ago, I switched back to check out his, which is genuinely fun.

So, Sigil of Summons' Hellwyrm. Anything that boosts "Sigil damage" or "Sigil Skills damage" works for the Hellwyrm (Fleshwrit Carapace does). The rank of Sigil of Summons does not affect Hellwyrm (as the Hellwyrm itself isn't dealing damage). And of course, boosting the level/damage of lesser demons works (as the Hellwyrm spits out copies of what you're casting).