Shrowser is the form assumed by Bowser when he unknowingly eats Elder Princess Shroob. In a manner of speaking, he is the final boss of Mario & Luigi: Partners in Time, although his battle is more like a minigame and is fairly easy, while the previous battle was much tougher and more "Mario & Luigi final boss" material. In appearance, he is Bowser flashing purple and green, often with the spirit of Elder Princess Shroob above him.
When a Thwomp elevator in the past sent Bowser flying back through a time hole to the present, he landed in Peach's Castle and was knocked out cold. Baby Mario tried to wake him up, but Elder Princess Shroob, who had been turned into a mushroom, went into Bowser's mouth, forcing him to eat her and transforming him into Shrowser to power him up. It is unknown why she did this, as Bowser was in control and unaware of Elder Princess Shroob's presence, but possibly she knew that the Mario Bros. were Bowser's enemies (evidenced by her attacking Baby Bowser on Yoshi's Island to make the Mario Bros. think she was on their side) and tried a final attempt to destroy them, or possibly there was a way for her to regain physical form, or simply she could not survive long in mushroom form.
Though the Mario Bros. had no chances to attack while fighting Shrowser, they countered and avoided his fireballs so they hit Elder Princess Shroob, which finally defeated Shrowser, destroying Elder Princess Shroob and turning Shrowser back into the unconscious Bowser.
The Shrowser battle is more of a minigame, as he gives the Mario Bros. no chances to attack. They can only dodge and counter his fireballs. Shrowser shoots fireballs into the air, which fall down toward them. These must be hammered back at Shrowser. He has 20 HP, shown as hearts at the top of the screen, with half a heart being 1 HP. Each fireball does half a heart of damage to him; Shrowser will deflect the reflected fireballs away unwittingly at Elder Princess Shroob. Sometimes he shoots bigger fireballs straight at the Mario Bros., which they must hammer back at him. It does a full heart of damage, as Shrowser deflects it at Elder Princess Shroob as well. He also shoots regular fireballs straight at the Mario Bros., which they must jump over. If a fireball is dodged, it will go to the left, then come back from the left and hit Elder Princess Shroob. Once all Shrowser's health is depleted, the battle is won.
Once the player gets used to Shrowser's attacks, beating him will not be hard at all. There is a save point before him that offers the chance of going back to before the battle with Princess Shroob.
- Shrowser is the second time Bowser is combined with a different main villain, the first being Bowletta.