Barrows drops work as follows:
You get X+1 rolls, where X is the number of bros (not Linza, she's not a bro) you've killed up to 6, so you get the optimal 7 rolls with and without Akrisae killed.
Drop rate is 450 - (Number of bros killed x 58) , up to a minimum of 73. So 1/73 for 7 and 1/102 for 6.
You then have 1/X chance to land on the table of any of the bros you've killed. Once the bro has been decided, each of his drops is equally likely (making specific Ahrim and Karil drops slightly rarer because they have 1/6 instead of 1/4 because dw).
Chance of at least 1 piece of Guthans with 1 bro killed: 2/(450-58)=1/196
2 bros killed: 3/(2*(450-58*2))=1/222
3 bros killed: 1/207
4 bros killed: 1/174
5 bros killed: 1/133
6 bros killed: 1/87
7 bros killed: 1/73
(source: https://www.runescape.com/a=63/drop-rates)
You can time your kills and then divide that with the drop % and you get the expected time with each tactic. I suspect it's best to kill as many bros as possible, but you never know.
Follow this guide for optimal abilities based on combat styles, then go to:
Settings>Gameplay>Combat & Action Bar>Combat Mode:
Now when you use combat, it will automatically use the abilities on your action bar.
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Bonus: Go to Settings>Gameplay>Combat & Action Bar>Action Bar Binding:
-Click the check box for both "Switch action bar preset when loading bank presets" and "Action bar binding", then specifically set your "Action Bar Binding Setup 1" to whatever you want to use. Here's mine as an example
This will automatically switch your action bar to whatever combat style you are currently using.
ex. If action bar 1 has your melee abilities, and action bar 2 has magic abilities, if you equip a magic weapon, it will automatically switch your main action bar to your magic abilities.
504% atm, consider looking into the drop rate because at 1/263 for a unique at 100% enrage isn't too bad imo. Expect many many deaths while learning, especial p5 but hopefully the upcoming practice mode changes would alleviate the frustration a little.