i did not found any caches except cache which been provided before yougo and seek for lyrium and officers, and you can manage to do all templar fights without using single healing potion on nightmare, and save all the potions for Envy Demon.
How to do it?
1. Positioning - need to fight in place where your flanks covered from archers.
2. Avoiding posititioning of several characters when horrors use their red aura.
3. Caring about hp/guard for main tank (Cass/Black) - red templars are easy with guard generation, generally , you can manage all templars fights in hall and sides with 5 regens on main tank, or even less.
4. Mage (better spirit tree Solas) should be in covered position, always back and unreachable/covered, but close to party.
5. I used 2 rogues to generate loads of combos. Sera as bow, full stealth spec lvl 7, Quizzie DW rogue with full stealth + poison. Intitate from stealh, and proceeed to combo. Combos are strong and work amazingly well at fight before the Envy demon when templars try to dispel the barrier.
Combos, Combos , Stunlock, : You can generate: Shatter, Rupture and as AOE - nightmare - which is incredibly useful because midn blast and well placed KP cone could put all the enemy horde into nightmare and send them into panic.
6. Prioritize squishy archers with rogues.
7. Do not care about hp on rogues during fights at all. They should not be hit. IF you lose one, you can revive with hands or spell. Barrier time is perfect for reviving. Revived Rogue can KP > Stealth > SB.
Once you go to Envy Demon, swap for your DW Rogue for bow. Till Envy turns into duplicate and Cole joins the fight keep running circles, micromanaging all characters to avoid cluster. Potions are necessay in some points if you screw some movement/ misjudge direction. Use Bees pots. You can keep running circles and ressing fallen mates, Solas is very useful bc of revive and barrier power. PW rogue also useful cause PW is actuallty strong way to damage. Tank should taunt and run too. Once Envy demon calls for red templars and Cole enters fight, you won already. You may need all your 8 potions on that Envy Demon first part fight, and no need to use those pots during templars fights at all, instead focus on positioning, timing and precious party combos, especially AOE nightmare. Nightmaring never gets old for me.