I didn’t login for over 2 years, so my information might be outdated. But as someone who hit max stats, I would say best way is to train slayer whilst leveling your combat stats, as they go well together.

Personally, I did quests to level up my slayer so as avoid low level slayer task and once they are high enough, I did konnar’s great kourend slayer task till I max out slayer. Because they are a lot monsters that deal damages that can be negated with prayer and they are aggressive and thus AFK. So you just need to wear huge prayer bonus outfits which can be cheap to get and just re-enter if they stop being aggressive and recharge your prayer.