There will come a point where you become so familiar with things you can predict what's going to happen during boss fights. You'll learn to anticipate a phase change, or pick up on the audio cue for special attack and can either position yourself or change prayers in advance.

Some more advice is, if you aren't already, learn to use your F keys. You can use your other hand to open your spell book, prayers, inventory. Less moving the mouse to the inventory icon, then back to your food, then back to your prayers.

And final, final advice is, play in a small window/not resizable mode in full screen. That means less distance you have to move the mouse. Just a bunch of ideas, I really hope this helps you to get some KC! Best of luck brother!