Ulcerava is a Solo Boss in MOW MY LAWN: TWEAKED
She can be fought after defeating Gemini, She is located at the Green Portal, which will lead you to her.
Defeat the archgoddess Ulcerava in combat. Little is known of the warden, besides that the archspear held in Ulcerava's hands once brought the barriers between universes down, allowing the beast Ultiel to escape her eternal bindings. - Ulcerava's Badge Description, The Fallen
Ulcerava is one of the Four Archgoddesses that you need to defeat to fight the first Final Boss of the game, She has 150000 HP, The server limit is 1 Player and 1 life, Defeating Her will give you The Archspear.
She wears black and green clothing, she wears some sort of mask and has green hair, and lastly she has a little back gun object on her back.
Assuming you have killed Gemini, you can abuse the distance of The Diamond Spatha to slowly but surely damage Her, if you have any other powerful weapons then this process will be easier. watch out for the green orbs that follow you, you can either wait for them to dissipate, or roll into them to make them disappear, without getting you damaged, although that sometimes is risky, since you could still get damaged due to the rolling animation ending, but in a smaller scale.