Soma is a large blue blob made of viscous fluid that can be drained out of the body. As it moves throughout the organ, it releases red tumors into the body, which must be lasered. When enough fluid is drained, the core will expose itself and leave behind a ring of tissue. The tissue must be drained or they will harden and need to be excised, while the core must be lasered within a certain amount of time to register an "Ok" before it disappears. After successfully damaging the core, Soma regenerates its fluid and the process must be repeated.
After the core has been destroyed multiple times, Soma will split into two. One Soma has a core in it, while one other Soma is drainable fluid. However, if the false Soma is not taken care of, it will burst and leave behind a large amount of tissue, which is more of a trouble to take care of.
Soma will periodically release a red tumor onto the organ which has to be burnt off with the laser. If left alone for too long, the tumor will harden then burst, causing vital damage and creating pools of pus. Then the tumor must be excised like any other tumor.
When Soma reveals its core, tissue will surround the core that must be drained, or else it will harden. Right when the tissue makes contact with the organ, it will start to drain vitals. If the blue tissue is allowed to harden, the vitals will plummet even more, and it must first be lasered, then removed like a tumor.
Usually after 2 or three successive OKs, Soma will split itself into two. The real Soma body will flash green in its center. If the Soma clone is allowed to explode, instead of a triangle, the tissue will split into a five-pointed star. In the X-Mission, if the Soma clone explodes, it will divide its tissue in a hexagon pattern and the tissue will harden faster.
When the Soma body splits, focus on draining the tissue it releases. Laser exposure to the core does not reset when Soma regenerates, meaning that even if you fail to register an Ok on the core, you will need less time to do so the next time the core exposes itself.
If the Soma core is only partially damaged, it is wiser to destroy the core first because it is partially weakened, and then drain the tissue, to save time.
There are two ways to tell between the real and fake Soma bodies - the one with the core will flash green momentarily, and the one with the core is the only one that will make red tumors. Use this to distinguish between both bodies, since the fake Soma must be taken care of promptly before it splits into tissue pieces.
Several other Atlus games, like the Shin Megami Tensei series, feature an item named "Soma". True to its name meaning "body", it would do something to the effect of fully restoring the HP and MP of the entire party.