[ACX]

In all seriousness..

First off, you need to scout it coming (I think you know when it's coming, if not, just tell me).

I believe immo/sentry is quite vulnerable when it first moves out because that is when the army size is smallest. I believe a macro hatch after taking 3 bases (don't know timing cos I'm not a zerg player) will help to pump out a lot of lings/roaches. You should preemptively set up a nice surround with your lings/roaches (some units left, some in front, some right) when the immo/sentry first moves out (note this is not a committed resistance) and future engagements. The aim of your first engagement with the immo/sentry when it first comes out of the base shouldn't outright kill the army (if it does, great!) but to wear it down by forcing them to use FFs and killing off key units such as sentries or immortals, and delay their force coming (giving you time to prepare, build up your own army, build spines and set up a nice surround). As a bonus, you could try and bait out FFs with your initial force.

In conclusion, I feel the best way to deal with immo/sentry is to:
- Harass and wear down the immo/sentry force when it first comes out.
- Delay their army coming to your base such as engaging very early (with not much commitment), counter-attacks into their bases (though they mostly block off when they move out), picking off lagging stalkers, sentries, probes, etc.
- Setting up a nice surround with your army to deal maximum damage and to force out many FFs as possible.

That is probably as much I know about immo/sentry all-ins by Protosses in Zerg perspective. I think the experienced Zerg players can help you with this more. Hope this helped!