Alright, I played the map from end to end. It was a few days ago but I left some notes. I enjoyed the map, although it was broken at times and I had to use some cheated items in order to proceed. I like the concept, and this map has great potential - but there are quite a bit of issues that have to be solved and lots of room for improvement. The core premise and general layout of the map works well! The issues are with bugs and balancing, which can be difficult to get right and can require some testing.
I like the overall layout with each level having a boss fight (well, cept the ocena level)
EoC was my favorite boss fight, I think. The arena looked pretty cool, you should light it up a bit more so you can see the build!
The spike area in the Corruption worked pretty well - usually I hate spikes in maps but I felt in this case it was dangerous enough to be exciting yet not too frustrating.
I liked how you included the meteor staff - since it's higher tier but has limited mana! It became sort of an OP burst fire attack with a long cooldown.
There were more accessories than slots, which I liked. Makes you switch out and prioritize different gear for different levels. This could be built upon in a future map somehow!
Balancing
Balancing in this map is quite problematic. You have the same gear for every boss despite them being of vastly different difficulty. The easier boss fights like King Slime and the Eye of Cthulhu are fun and balanced, but facing Golem, Plantera or the mech bosses is pretty much impossible, especially since it's expert mode. Fixing this would require quite a bit of work.
One way to solve this is to allow the player to unlock new armor sets, accessories and weapons for every level they complete, so if you complete the easier bosses you get rewarded with better gear. Another solution is to lock every level into tiers so you have to complete all the easier levels before reaching the harder ones, unlocking new gear in the process.
Bugs
There are quite a lot of bugs and issues in this map.
Don't worry - most of them are easy to fix. Here are all the bugs I discovered playing the map.
At the start of the map, as stated previously, there's a goblin army. This really hurts the map for obvious reasons, the spawn room turned into a bullet hell pretty much. Just go onto the map, defeat the army and reupload it.
The played detector in the first door is a little bit off since you can get stuck in the door. If you move it a block to the right it should work properly.
There is a second cave in the crimson area, from which you can access a big naturally generated cave system. This was pretty confusing and it didn't seem intentional. I'd suggest blocking it off.
Unless I did something wrong in the crimson cave, there are a couple parts of the cave (the ones with meat blocks) that are too narrow to pass through. Simply widen the tunnel and it should work out.
In the ocean area the traps don't work properly. This is because timer blocks shut down when you leave a world (common and very annoying mistake). To fix this, you could somehow force the player to activate the timers when they enter the area, with some sort of lever or pressure plate.
The trap in the hell area is reversed. The blocks appear when you stand in the detector instead of disappearing. Should be pretty simple to fix.
The teleporter to the heaven area doesn't work. I cheated some items in to reach the area anyway though
The heaven area works well, but the statues don't seem to spawn slimes properly. No idea why that doesn't work.
The teleporter to the bee area doesn't work. I haven't visited this area but I cheated in the gem.
The jungle area doesn't spawn Plantera if I recall correctly
The teleporter from the final arena to the ending area doesn't seem to work either.
Other issues
Some areas, for example the teleporter hub, are very dark. You should light the map up, or perhaps supply the player with some torches!
Some boss arenas, such as the Eater of Worlds and the Golem arenas, are very small which makes the bossfight very difficult. You should enlarge these arenas and the fights would be a lot more fun. And the spikes in the EoW arena makes it even harder, I think they should be removed.
Some areas have natural chests available to the player. This doesn't seem intentional and could potentially break the balance of the map. For example I recieved a magic mirror from a chest which allowed me to teleport back to spawn when I had a gem. But to be honest I think a magic mirror could be good to give to the player anyway.
Fluff (very very optional)
Loki's set is one of my favorites, but I would love it if the starting chest could include some more vanity sets and dyes so you could customize your character! Or you could hide cool vanity items in secret chests around the map
secrets are fun!
I like when a map has a story to tell. If you want to, you could add some lore to the map that explains why we're hunting for these gems. Only if you want to of course!
Summary
Right now the map is kinda broken but with a little work I think it can turn out great! Just fix the bugs and play through it once or twice to make sure everything works like it should. Then, work on the balancing of items to bosses so they aren't too difficult.
Despite all this I really really hope you don't get discouraged! It's always nice to see a new mapmaker in the community. There aren't too many of us and I hope to see more from you - if you want any advice or help, hit me up anytime!