For issues you find with the Switch and Console releases, please follow this link and give as much detail as possible. This is the speediest way to get info to Pipeworks and get a hotfix in the works.
For issues you find with the Mobile 1.3 update, please follow this link and give as much detail as possible. This is the speediest way to get info to DR Studios to look at your issue. Also, some troubleshooting hints here.
What is the hardest fight that you’ve ever fought?
Right after defeating destroyer, I went down to underworld in my trusty ranger cobalt armor and got confronted by a red devil. After about 2 minutes avoiding the tridents, I finally got killed by a hellbat.
Expert Skeletron is pretty high up there. It took about 5-6 tries until I beat it the 1st time and even in new playthroughs it still takes a few tries before I can defeat it also requiring to get Molten Tier Armor in order to stand a chance.
Another one is the Lunar Pillars in whole. It is still challenging even with endgame gear and the 1st time on Normal Mode I fought it, I died many times. Not to mention the Moon Lord that follows it which is a difficult fight in itself.
But the hardest fight for me personally was either doing the Pumpkin/Frost Moon without Duke Fishron or Endgame 1.3 Gear or the Mech Bosses in 1.1.
I couldn't get past Wave 4 on my own without Fishron Gear back in the early days of 1.2.2. Whether it was because I had an improper arena or didn't have a good strategy, I'm not sure. And even when I got Fishron Gear I could make it to Wave 10-13 in both the Pumpkin/Frost Moons, but still couldn't clear wave 15 until I got Lunar/Moon Lord Tier gear in 1.3.
I never defeated the Mech Bosses legitimately back in 1.1. Very likely because I wasn't nearly as experienced as I am now only having 100-200 hours of gameplay compared to 1000+ now. They aren't as difficult now even in Expert Mode, but that is likely because there is greater equipment variety to choose from compared to 1.1 allowing you to get a better style for your own tastes.