From Grit Flowage, you enter Sedimentary and destroy another four ladders and four teleport beacons. The Xtraction point is near the exit. Stock up on health and energy packs (if you need them), get your team set for a huge boss battle (change out psychic X-Men for energy or physical X-Men), save your game and then head through the door to Dechlorination.
Powers: Throws bone projectiles in a 90-degree arc
As the battle begins, quickly take a flier on a circuit of the outer rim of the room. You're looking for a DR disc, Challenge - Gambit. Once you have it, quickly return to the fight. Each time you beat Marrow down, one of
Marrow's Lieutenants pops up in one of the corners of the main stage and revives her and all her Leviathan henchmen. Killing the Lieutenants when they pop up will shorten the fight; however, you can still beat Marrow down without taking out the Lieutenants. Each time she is revived, she has less of her full health, and she will eventually not be able to get back up.
When Marrow gets low on health, quickly run over to a corner and look for the Lieutenant. Iceman is a good X-Man to have here, as he can slow, or even freeze, the Lieutenant and allow your team to get over for the kill.
There are four Lieutenants, so you're going to have to fight Marrow and her "Gene Nation" at least five times. It is recommended that you have purchased some Xtreme powers and saved all your tokens so you can, at least, dispose of the henchmen easily.
Once Marrow is finally finished, the mission ends in a cut scene and it is either on to the next mission or back to X-Mansion.