OSRS Tombs of Amascut - Keep Back made easy! (see description for extra info)
In terms of mechanical difficulty keep back is comparable to 6tick red-x'ing which most people pushing high raid level will know how to do. This guide will offer some basic tips to help you learn the invocation.
EDIT I:
For those who prefer a visual cue over counting ticks, stepping back as the projectile hitsplat disappears should do the trick. Beware you have to be quick or you will get meleed.
EDIT II:
This tip is for advanced players only: tick loss with fang can be prevented by flicking melee when his attack tick lines up with yours. You can resume step-backs after flicking one hit if you flicked the correct hit. Beware you will take more chip damage than doing step backs every time.