This challenge guide is brought to you as a collaboration between Arknights Youtuber Eckogen and GamePress. Eckogen's Video can be found below, and his YouTube page can be found here.
If you are looking for the guide of the normal version of this Operation, or other HX guides, you can find them here:
Corrosive Tiles - Operators deployed on these tiles have defense halved.
Challenge - Deployment Cost of Ranged Operators are tripled.
Note: There aren't really ranged enemies on this map outside of the corrosive slugs, which makes it easier for the medics to focus on healing defenders.
Vanguards are the essence of this challenge since three times the normal DP is required for ranged Operators (Casters, Snipers, and Medics). In general, the vanguards themselves are pretty interchangeable, but try to have them at E1 and leveled some for an easier time.
Low-Cost Ranged Operators are especially effective since the difference in cost post-tripling really adds up. So long as the DPS is high enough, lower rarity Operators will offer much more wiggle room compared to their shinier counterparts.
If there is always barely enough DP to go around, the first cut would be using lower rarity ST Snipers like Kroos, Adnachniel and Rangers - Kroos/Adnachniel doesn't require Elite 1, and they are very cheap to raise. Of these, at least one ST Sniper needs to be E1 and levelled, preferably Kroos in this case.
AoE Casters are hit the hardest due to their inherent higher costs, but only roughly one is necessary. 12F is notable due to his 50% physical dodge and lower cost providing breathing room.
Given how Medics are especially at a premium, using Heal Tanks could be useful for some strategies.
Given these considerations, the suggested team comp for the strategy is this:
3 DP-Recovery Vanguards
1 DP-on-Kill Vanguard
3 ST Snipers
2 Medics (+Lancet-2)
1 AoE Caster/Sniper
Although the enemy layout is the same, there are roughly three major phases for this battle: Vanguard Deployment, First Wave Deployment, and lastly Medic Deployment.
Start off by placing Vanguards aggressive on the uncorroded tiles at front, with the Recovery Vanguard at VR1, and the Kill-Vanguard at VK1. As soon as possible, place the other two Vanguards at VR2 and VR3. Deploy the strongest ST Sniper at the top facing down (ST1), and then deploy Lancet-2 near the front Vanguards to heal them a bit.
The next phase begins at around the 14-kill mark. Deploy the second-strongest ST Sniper near the bottom Vanguard at ST2, followed by the last ST Sniper facing left at ST3. As soon as possible, place down the Defender in the middle of the Snipers at D1.
Retreat the top left Recovery Vanguard after they grant DP, and then deploy a Medic at M1.
The 58 kill mark is the next major checkpoint, where DP will be more scarce. Retreat the other Frontline Vanguard as late as possible, and then deploy the AoE at "AoE". As soon as possible, retreat the ST3 and deploy the second Medic at M2.
At around the 80 kill mark, the giant wave of Corrosive Slugs will spawn. Proc as many skills as possible at this point, especially for healing and defense. This formation should hold out for the rest of the battle.