One last thing to remember: Utilize abilities as often as OSRS gold you are able to. When I cast Fire Surge on an glacor and make use of a magic ability I will not use any arrows at all. The same applies to arrows that are within the range. For slayer tasks, you'd want to avoid ultimates, and focus on basics and thresholds, until you can use Guthix' Blessing to heal, or Berserk/Metamorphosis for some crazy damage.
In the past, I've explained nearly all you need to learn regarding the EOC for you. Perhaps I'll write an article on this. If you require assistance, please contact me hereor on the games like "Renegaderp".
If you aren't able to get an official crossbow, you should have an idea of getting the Armadyl crossbow. Always make use of dual weaponry when slaying tasks. Choose the most powerful weapon you own. If you have a lv 80 ranged wep as well as an lvl 80 melee weapon as your top tier weapons choose the weapon that the monster is weakest (according to the combat triangle).
There is no need for anything other than bandos or barrows to complete slayer missions. If you need weapons that are lvl 80 I would suggest to purchase chaotic weapons and spend your money on other items. I believe that people should only opt for the lv 90 items if they have cash to burn. It's up to you, however.
In terms of bolts, you can now put any bolt on any bow. The compatibility issue is gone. If you are using level 80 royal bolts in conjunction with buy osrs fire cape an lv 70 crossbow, the bolts will be equipped with a damage cap similar to levels 70 bolts. If you own an offhand bow that is level 60 the mainhand bow will be capped at lv 70 bolt damage, while the offhand bow will be capped at the level 60 damage. However, I do purchase Royals because they're the cheapest and it's not really a big deal in any way.
