Finally, a Risk 18 clear in my account. I think I combined 3 different strats I saw.

The base strat is a Risk 18 clear video using a caster-based team translated by Dr.Arknights here: https://www.youtube.com/watch?v=zlrDUcDknXk

Even though, I have all the units in that video, I struggled to clear Risk 18, so I tried to find other inspirations.

I stumbled upon a video that I can't find anymore, that details using an E2 Nightingale to tank the casters' attacks, which looks safer than having to rely on Firewatch's invisiblity. The Nightingale in this video is actually just E2'd before I attempted the run.

I was still stuck, somehow, and continue finding inspirations, which is where I found MrPantaloons' video ( https://www.youtube.com/watch?v=NvZRwJAxj7M ) where pinning down tough enemies is accomplished using Angelina's skill 2. Angelina's skill 2 is more reliable in pinning down a single enemy, which I badly needed.

Combining the concepts from those 3 videos, I devised a plan. Most of the operators on the left half is retained from the first Caster strat video, with the exception of Glaucus being replaced by Angelina with skill 2 (from looking at MrPantaloons' video). The right side is replaced by a Nightingale + Sora + sniper strat from the second video (that I now lost).

Gravel + Projekt Red is a great combo to keep the Invisible Caster exposed for the longest time to give time for Exusiai to kill the caster, but most of the time, Exusiai with S1 deals enough damage that I usually only need to deploy Gravel.

Risks taken: Max HP Seals are reduced to 1 All enemies gain +50% DEF Guard and Defender operators cannot be deployed All friendly units have -75% Max HP Natural DP regeneration rate is reduced by 50% All enemies gain +80% ATK All Armed Militants gain +80% Max HP, +40% ATK, +50 RES, and +2 weight levels All Avengers and Hateful Avengers gain +200% DEF, +120 Max HP, and +80% ATK Max Squad size is reduced to 10 Operators