Wait, maging in melee gear? isn't that just simply be splash. Also, are there any quests I need to RS Gold do other then preist in peril? (I must finish it quickly!) The precision of Slayer dart isn't affected by the magic bonus, it just affects the strength of your hit with it , so it's best to use melee armour for better defensive bonuses.
The accuracy of Slayer Darts is affected by the mage bonus (go and kill metal drags with the rune and compare it to Mystic and you'll find an enormous difference). Damage however, isn't. You will hit the same in mystic, as you would in a rune.
For Black Sally really only useful if you have a high defence level or are using barrage/blitz as well as to dodge. Sally is a little more reliable than slayer dart and, according to my experience, hits more at 70 mage then slayer dart hits at around 70 mage. If you're not blessed with 85+ def, or the magic level is 82+, you might as well just employ slayer.
The 78-80 attack shouldn't take any time at all, perhaps two days, or one week if you work slowly. I personally prefer to train on Green Drags, or Blue Drags in Taverly Dungeon if you uncomfortable in the wild. It's pretty slow exp. However, it's decent in terms of cash flow.
If you're after more experience. and speed, I would go for Yaks, Experiments, Flesh Crawlers (SHoS), or possibly Rock Crabs. Each of Buy Old School RuneScape Gold these options will offer a steady flow of exp. In addition, they are being fairly casual and low risk.