Practice makes perfect
Before move to high level PVM, it is wise for you to try some easier bosses and practice to work your way up and become more skilled. When it comes to solo PvM, you can consider tackle bosses like Zammy or Bandos or even Dagannoth Kings in a duo or small group. After learning a couple bosses the others will become easier. You will be able to gain the hang of the mechanics and move to solo.
However, you should keep in mind that upgraded Barrows, Kalphite King, and Vorago are bosses that are impossible to solo due to the various game mechanics. It is worth to point out that it is not wise for you to go into a PvM scenario in hope of getting hundreds of millions. Just as you can see, the average GP/h is just that: an average. It's really the absurdly expensive gear that drops at an insanely rare rate.
Bring out a second action bar for your utility
You can also consider bringing out a second action bar for your utility and support items or abilities such as food and defensives. Besides, you can place Food, Prayer potions, Freedom, Anticipate, Provoke, Debilitate and Surge/Escape on the bar and bind each slot to a comfortable/spammable key. Just as you can see, this will help you use those keybinds quickly without worry about you're combat action bar, and also this free's up space for more abilities useful against your current target.
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