Since they only come around once(?) per month this is the first time I have been at an appropriate level to try the ship tiers of a Galactic Challenge and it is way too hard.
I am trying to do the Tier IV challenge against a level 70 fleet. I am level 82 and running the three Geonosian pilots with Vulture Droid as a reinforcement (7* Sun Fac and Spy, 6* Soldier, 5* Vulture with all pilots at 7* and G9-10) with Home One. It is a complete slaughter--I am lucky to kill one enemy ship before losing. The game doesn't let me see the opposing pilot or ship stars, ability levels, or gear levels but it seems likely that they have inappropriate gear and/or ability levels for their level.
I am not saying that Galactic Challenges should be a cakewalk but it should be possible to do level-appropriate content with a reasonable squad and you cannot right now.