With the new feature introduced for bosses, they were allowed to switch pokemon. In this case, attacking with stat-destroying moves in hard level boss battles was useless. 

+400 e.v. It's impossible to control a Pokemon with stats without a stat reducer. The carrier is also useless as there is not enough environment for the buffer pokemon.

It's not impossible to win boss battles without the old system, but it's very difficult. Double bosses are IMPOSSIBLE.
 

Fix boss update please...

They are not impossible since many players still do them, so that statement is simply wrong. There are still multiple strategies to defeat them if you prepare properly.

I need a tactic for Medusa and Eldir Hard fight...

There's nothing to fix, nothing is broken, quite literally, just get good.

16 minutes ago, Raika said:

There's nothing to fix, nothing is broken, quite literally, just get good.

Its more like it was broken prior to this fix.
When you come to think of it, the AI was pretty much dumb... and now its smart but dumb at the same time.