While playing expert mode on parallel realms the last one on 2/3 is corrosive agent black Dahlia that character literally never dies it's not a fun game when it's straight up a character who's immortal and you can't even win it because that character keeps getting more heath added.
She can be hell at low level, but becomes very easy with the right counters. If you don't use a counter, she can be unkillable indeed.
First thing: You want to consider not using blockbusters if you don't have good counters. And no critical rate (unless you use slime debuff as you can read below).
When you don't have specific counters, the best characters/variants I used in the beginning myself are Beowulf with 2 wulfshoots (Wulfsbane is the best because he inflicts heal block and hits hard) or Cerebella with merry gorilla loop and pummel horse to prevent crits (Blitz and Glamour works best because of inverse polarity, as well as Harlequinn for easy gorilla loop). Also, any critless Eliza with special moves works, with Chaos Banish special move as well of course. Any critless character with only special moves works, but those three are some of the best for this job. Again, don't use blockbusters.
Now for specific variants that can work are counters:
- Phantom Wrangler : for a silver variant, she WRECKS all defensive Dahlias with the easiest way to inflict slime in the game, especially when you use hop to it (the dust bunny special move). This is your go-to early game imo. Have a high crite rate for a better result. Other slime users work as well but not nearly as good as Wrangler.
- Purrminator: requires high Crit rate (around 70% is good), but she is one of my favorite and most fun ways to deal with her. She starts weak but then becomes overpowered. You do need to use the Circuit Breaker blockbuster to reduce Corrosive meter though, or just intercept her which is harder, otherwise she may kill you with a BB3.
- Gust Buster : easiest buff steal in the game, and very fun! Available in monthly Marie Gold PF.
- Silent Kill: buff removal but a little bit weak in high streaks, and not as consistent as you'd want her to be
- Mean One: best counter in the game because she replaces all buffs with debuffs and drowns her to oblivion with one Argus Agony BB
- Rainbow Blight / Red Velvet / Heart of Darkness / Death Wish / Heartless: best HEX users and one of the better consistent ways to deal with Corrosive (especially Death Wish).
- Xenomorph / Dame Slayer / Space Case: best Doom users
- Stand Out / Ogre Achiever : requires high Accuracy, and they just prevent buff usage
- Unholy Host : removes buffs and inflict curse, but can't use BBs since Unholy nerf
- Rusty: She has a chance to remove opponents buffs and replace them with bleed. She doesn't work very well because bleeds get replaced by heal anyway, so a critless rusty...? nah
- Nunsense: notorious cheapest anti water defense, but requires you to stay in air element. She gets so much buffs she may beat Corrosive by brute force. It doesn't work on high streaks PF or high difficulty Parralel Realms though because her attack is low and HP modifier is very high.
- Doublicious: steals buffs, but can be a bit inconsistent
- Shadow Ops (also Purrfect Dark or Dark Horse): Curse, but can't use blockbusters. Shadow Ops is the better one for Corrosive because of her kit imo, which makes her able to use her Canopy Bounce BB.
- Soul Crusher: preferably with no BBs, can easily overpower her with deadeye and her SA2
- Unfazed and Vaporwave Vixen: Miasma cheese, but can't use BBs. Vaporwave is better because you don't need to hit her. Just use bursts and taunts to kill her with time.
- Djinn Frizz / Class Cutter : brute force with the drill BB to remove buffs, as well as heal block from Class Cutter with Drill special move cheeze
- Phantom Threads: removes buffs, but can't use BBs
- Furry Fury: buff removal and brute force
- Indomitable: Miasma and buff steal, but she is exclusive and is a bit risky to use
- Summersalt: Buff removal and brute force with her kit, but not very effiscient
- Psykid (exclusive) : inverse polarity but can't use BBs
As you can see, lots of cheap variants and PF variants available to use, not necessarily rare Diamonds or Golds.
Of course, there might be other ways, but not as efficient (like buff removal BBs or special moves from some other characters. Like, one very gimmicky way is to kill a bronze while having Dark Might to inflict a permanent curse on her. But very specific strategy for one variant isn't optimal hah.