This is not the most efficient way to OSRS gold go, but is effective for me. My numbers are 99 attack/97 strength/95 Defence/98 HP/80 Prayer/90 Magic/80 Ranged therefore if you have similar stats , it's a piece of cake. (But I'm no expert :P If your stats match mine, there are much better Barrow strategies.
Wait, maging in melee gear? Wouldn't it be just a simply be splash. Additionally, are there any other quests to complete that aren't preist in peril (I better finish that one soon haha!) The precision of Slayer dart is not affected by your magical bonus. It just impacts how hard you strike it . Therefore, it's better to stick with melee armor to get better defensive bonuses.
Slayer Dart accuracy is affected by the mage bonus (go kill metal drags in the rune and then compare it to Mystic and you'll see that there is a significant difference). Damage, however, is not. The same thing happens in mystic as you would in a rune. In regards to Black Sally really only worth using if you have a good defense or are using blitz/barrage as well to buy RS gold sidestep. Sally is a bit more consistent than slayer and, i believe, it is hitting more frequently at 70 mage than the dart slayer hits at 70 mage. If you don't have 85+ def or an 82+ magic score, it might be better to use slayer dart.