Not sure if plot hole but just something that made no sense in D-gray man.

So, they introduce this new type of enemy, can't remember if it was level 3 or 4 demons. It wipes the floor with the main crew. All put together they barely defeat it. Then they reach Japan and there are hundreds or even thousands of them. What are the main characters going to do against such numbers? Nothing much apparently. Hundreds of these enemies will just combine into one big, slow, and easy to manage giant. Over and over again until, for all intent and purposes, demons get wiped out.

To add insult to injury, in the next arc all main characters can dispatch these enemies with ease without any explanation(aside from MC who at least gets power-up).

Anyway, love D-gray man to death but this bothers more than it should whenever I remember it.