Can't do "Hope on the waves"

[Iscah]

NPCs in HW and SB quests count as party members, so you can use your party-only abilities on them. Intervention and Passage should help you out. Though "kill the thing before it kills you (or your NPC companions)" is usually best. Sword Oath and DPS combos! ALL THE SLASHIES!

I just completed the Lv60 end-of-Heavensward SCH quest battle last night on my alt, and I don't think this is the case in all circumstances. Succor (AoE healing/shield spell) didn't seem to apply the shield, though single-target Adloquium did. But then the "spread Adlo to party members" spell didn't spread it.

I'm pretty sure the single-target abilities could be applied in ARR as well.

Either way it seems closer to how you can interact with alliance members. You can target a single player from another alliance for a direct spell (certainly for raises, not sure about status effects) but AoE healing spells will only affect players on your party list.