I think you should always train with RuneScape gold slayer, it shouldn't take too long to receive 70 whatsoever; maybe a couple of days depending on the length of time you play every day. A great set up for slayer which is fairly cheap would be to use - Slayer Helm or even Neitznot (sp?) Helm until you get the slayer helm, Amulet of Glory, Fighter Torso (can readily be acquired in a few hours), Torag's platelegs or Verac's Plateskirt if you will be using prayer in your tasks as it supplies prayer bonus, Whip /Dragon Scimitar, Dragon Defender, along with Dragon Boots, and Obsidian Cape (unless you've got a firecape or skillcape).
Bear in mind that is simply a proposal for equipment, you always have the option to switch things up. By way of example, you may swap out of your guardian for an Obsidian shield should you really feel like you want the extra defensive boost, and you could always use dragon plateleg's if you don't want to be repairing your barrow's armor because it degrades in combat.
Furthermore, if you do not have the pursuit requirements for the helms, then DO THEM. If you plan to do any slaying whatsoever you ought to do the Smoking Kills quest as soon as possible. Do not be afraid to try different things and Old school runescape buy gold find out what works for you.