Where is the best place to kill trolls for Slayer while ranging? I have access to the Neit trolls.

You could safespot mountain trolls on the Death Plateau.

bad idea Mountain trolls have high range defence, it won't really matter if you safe spot them, but it takes a while longer to kill them with range then melle would

Cannon those ice trolls and equip rune c'bow with broad arrows to help out your cannon. A task is usually done in 30 minutes or less. There's a guide on the forums here somewhere, so you could search for it.