I don't have a champ with heal reduction, so i tried the hard way .. a champ with curse set for 50% heal reduction and a champ with destroy set.
the problem is that on spirit keep lvl 12 the boss heals herself for ~75k hp and with 50% heal reduct ~37k hp at her full health, after some minutes when she had 40% of her max hp destroyed her heal was the same... WTF ? destroy set is supposed to reduce her heal according to her max hp.
Some sets have garbled or inaccurate descriptions.
1. Poison set works only per skill, no per hit. For example Athel A1 has just 1 chance to put poison with this set, not 3 chances.
2. Destroy set counting only dmg took per specific enemy. So when you have Battlesage with this set on Giant Spider dungeon, than her hits tooks off only dmg dealed to GS not counting his minions.
3. Speed, attack, defense, life sets giving % from the basic amount of stats. So if you have for example 1000 (basic def)+ 1800(additional from eq), than it gives you just 150 def, instead 420.
So maybe in your case there is some hidden issue too, you'll never know until you find it out.