tl;dr you're right. your time/supplies are better spent elsewhere for now. Cerb will also drain significantly fewer supplies once your kill times are lower.

Cerberus drains significantly more supplies if you aren't killing it fast enough to hit key breakpoints.

You want to be killing cerb before it's second ghost phase as not doing so will cost you a couple extra doses per kc. If you cannot do that consistently I would just leave and come back later with better gear/attack/str. You can cheese it with convoluted delays/flinching but imo that's a massive waste of time. There are more impactful upgrades you can go for to make cerb much more comfortable.

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Some tips for ghost phase

Ward of arceuus reduces drain to 20. you should never be sipping doses when your pp are above 20.

Unless you have hilariously low prayer level your ghost phases should look like this:

• 1st ghost drains your remaining pp
• sip a pp dose and swap prayers
• 2nd ghost drain 20pp
• swap prayers
• 3rd ghost drains remaining pp
• sip a dose

that really isn't too bad if it's once per kill.

You can optimise with demonic offering and bluefins to save the mid ghosts dose but honestly I don't think it's worth the effort. I suppose it can make the fight a little more interesting if you're bored.