If you can't isolate the main drain, you won't be able to use the pool pump to drain below the skimmer level. Once the skimmer goes completely dry, the pump will just suck air and lose prime and overheat and possibly get damaged. You'll need to rent or buy a pump to get below the skimmers unless you are able to completely shut off the skimmers and only draw water from the main drain in the bottom of the pool.
I'm looking at your post history. You said in another thread that "My fill water is around 375-400 CH." Are we speaking the same language? Fill water is what comes out of the faucet. Pool water is the stuff in the pool with bleach and stabilizer in it.
If your pool water is only 375-400, don't even bother draining. Not yet. My CH has never been that low in 4 years.
Let's say your pool water is 400 CH and the water out of the faucet is 100 CH. If you drain 25% to get to the magic 300 poolmath says, it won't work. .75*400 + .25*100 = 325. If the fill water out of the faucet is higher, say 150 CH, then it's going to be even worse.