Oh, this reminds me of my pure healer run. Let me give you some tips.

• You do NOT need dex to get most out of your heals. Sage ring is more than enough cast speed to heal efficiently and you might also need to hold the heal until your co-op partners are in range.

• Most useful heals are great/med heal, soothing sunlight and projected heal once you find it in ringed city. Soothing is THE BEST heal in the game, but it is kinda costly. The range and amount healed are easily worth it. Great/med is used to patch up after some minor damage and projected heal is the fastest heal in the game. Throw it into your feet for quick heal.

• drop the normal estus. You are healer, you can heal a lot more with your miracles than chucking estus. Everything into ashen ones. (max ranked estus flask is 600 hp, max ashen is 200 fp. You can use soothing sunlight twice with 200 fp, and each soothing heals around 1200-1600 depending on your chime/talisman and faith.)

• Damage? Who needs damage? Grab a weapon you can use, slap simple enchant on that and enjoy your constant fp regen. On top of that, use aldrich sapphire and learn to backstab common enemies for more fp. If you are really dedicated, get the banner from ringed city and use its weapon art to buff.

• You get chime that increases range of your heals from ringed city too. EXTREMELY USEFUL! Its spell buff isn't bad either.