I thought the enemy levels are based on the max league you achieved in the last season... or is it based on the max league you've ever achieved?

In the past I remember dropping league so I get easy players in the colosseum, but since I reached L1, this doesn't seem to work anymore. Now I can only choose from L5, L3 and Dirty League level players. It's unplayable.

They applied an update that changed the rules to prevent killer smurfs.

It works off your highest ever recorded rank in the tower.  I tried dropping down to Level 11 and staying there until the weekly timer ended. I didn't raise my level until the next weekly event started but was still presented with my best level-4 as my soft option.

I changed my tactics: I've delayed progressing up the tower so I can farm new cards.