Onyx
Banned
They chased Bela from town to town with little opposition for a long time (but failed to catch him) and they also took and burned Zagreb. They didn't have enough men to storm Klis or other Dalmatian towns and definitely not the islands, which is where Bela hid out until they left. But in general I think their forces were just too small by '42 so they met with lots of difficulties in the Balkans (Bulgaria as well as Croatia) where ambushes could be effective now and then.
I don't think it's a good general recipe, just somewhat effective for that very specific situation where there's few of them.
Mongols also invaded Serbia, but I think it was more of a raid than invasion, but they were defeated. However Stefan II Milutin gave Khan Nogai his son as a hostage for a while so Serbia can avoid anymore raids from the Mongols when Nogai threatened to invade again
In saying, the Balkans are going to be hard for the Mongols considering the geography