Again, I am going to split this up into two. The regular trolls and the ice trolls have pretty different stats despite being of the same race and looking so much alike.  The regular trolls are only weak to melee, their defense bonuses for those attacks are all 0. Their bonuses against range and mage are both plus 200. So you will not hit them very often with attacks based on those two skills. Melee will be your only option here. Despite a cannon being ranged based, it will hit pretty often, and I would still bring it if you have one and have the money needed for cannon balls.  Ice trolls are weak to melee and magic. This gives you the option to safe spot them if you want to use magic. This is a good tactic if you are a low level or if you do not have the prayer level needed to pray melee.  When you are killing ice trolls, I strongly advise you to bring a cannon. A cannon will help you kill these monsters very fast and easily. There are spawn points where about 5 to 6 of them spawn. If you do this that way you can get more than 200k in combat exp in melee and ranged combined. You will also get about 45 to 55k slayer exp an hour, depending on your stats and concentration while killing them. So in short, the mountain trolls are weak to melee but a cannon still works. The ice trolls are weak to both melee, magic, and a cannon also works very well on them.