In order for the quest area to register me killing the Banshee mistress I have to deal at least one lifepoint of damage with actual combat, a recoil doesn't count as your hit. So I had to use Air Rush when she was really close to death, else the kill wouldn't count.
I decided that I'm going to quit after I max out in a week or 2-3 (not sure when I'll host the party), so I won't be doing Slayer anymore. If I ever decide to get back to Runescape I'll probably try to get a few Slayer points for the task blocks and Helmet.
First of all: Amazing goal, I do hope you achieve it; you have my entire support. I'd love to attend your party, as well.
Also, I'm not quite sure. When I train combat, sometimes my Constitution just goes up. How are you holding it down?
Whenever you're in combat you'll get Hitpoints XP, there's no way you can stop that. (the only combat that won't give you Hitpoints XP is a Dwarf cannon) I trained my melee statistics with Soulwars, to avoid getting Hitpoints XP, and I trained Ranged and Summoning with a Dwarf cannon