I was playing through The Rotting Metropolis and expected a major miniboss fight at the end that would take a while and nearly kill me (if not just kill me). After 1 minute I was standing, unharmed, in the middle of a large goo pile from what once was a Toxilargo.
That thing was very easy. I was playing on elite with 4* equips, solo, with no auto revive and very low HP when I reached that arena and still killed it easily.
If you want it to really be at least somewhat difficult, give it the dash of Giant Lichen Colonies. The dash is the entire reason GLCs are hard. Remove the dash, and (almost) no matter what else you give it, it will be easy.
How about instead, let it spawn in mini slimes plus allowing it to launch wave of spikes like a retrode beam, but instead summons spikes from underground. This would definetly make it difficult sense the spike would be able to go long distance making it be able to launch attacks across the whole field. The spikes does piercing damage and poison status.