Just incase if you got the same pokemons as I do or similar, you can use Brionne (leftovers) since it can tank. However, you'll need deoxys to keep lowering its sp atk otherwise it'll be much harder. Lead with deoxys holding choice specs then switch into drowzee since he can actually tank, also it don't matter what move you use with drowzee since his goal is just to make sure deoxys can switch in safely (that's why I won't mention anything about the moves however you can use confuse ray or any sort of debuff moves if you want). after deoxys got in again switch to Brionne it'll definitely tank and outlive frosslass phase 1.
When you pick popplio it is already as perfect as it can be (liqiud voice with hypervoice you can buy TM from viridian city) nature is also bold iirc so just evolve it.
Phase 2 gets bullshitter but for me it was easier cause I had the crackhead minior (choice banded with 252 atk + 252 speed adamant nature) just stone edge the mf and she dead dead although let's start from the beginning of phase 2 after you just beated phase 1.
After Brionne outlives frosslass it'll start another battle however it can mega evolve now and it means it lost the stat changes which you previously hit her with. You lead with deoxys again switch to Brionne let her die then switch again to deoxys after another mon dies. Ok in short you need to debuff her 3 times before you can safely go to minior (let deoxys die if you're done debuffing or do whatever you can with it I guess like protect to go defense form so you can chip damage froslass before dying cause after it mega evolves it's faster than even a maxed EV trained deoxys apparently.) cause her attacks are too weak and 2-3 stone edge is enough to pulverize 70% of her HP. I say 70% HP assuming you going through those switch ins you managed to chip some damage despite her soul siphon.
I also assume all of your mons are the right nature and max IVs also EVs. misdreavus holds choice scarf because if it were to put on gastly itll be slower speed wise against froslass at least I think so. That's how I beat it not sure if it'll be helpful but goodluck