Wait, maging in melee gear? wouldn't that just simply be splash. Furthermore, are there any quests that I have to complete that aren't preist in RuneScape gold peril? (I should finish that one quickly!) The effectiveness of Slayer dart is not affected by your magical bonus, it just affects how hard you strike it . Therefore, it's better to take melee armor to get better defensive bonuses.
Slayer Dart accuracy is affected by your mage bonus (go kill the metal drags in rune , and then compare it with Mystic you'll be able to see the vast difference). Damage however, isn't. You'll be hit with the same in mystic the way you would in rune. If you're looking at the black sally, it's only worth it if are able to defend well or are using barrage/blitz as well as to sidestep. Sally is a little more consistent than slayer and, i believe, it is hitting more at 70 mage then slayer dart hits at around 70 mage. If you're not blessed with 85+ def, or 82+ magic, it might be better to employ slayer.
78-80 Attack should not take long at all, maybe a couple days, possibly an entire week if your progress is slow. Personally, I would train at Green Drags, or Blue Drags in osrs infernal cape Taverly Dungeon if you uncomfortable in the wild. It's pretty slow exp., but it is an acceptable cash-wise.