Recommended progression order

First boss
First boss is the Royal penguin. He doesn't drop anything special, but i like it. You can summon it by the Fish cookie

Impact
So, second boss will be the Impact. You can summon him with Redstone controller used on the piston in the nether. Use ender pearls to avoid his attacks and hit him with range weapon. He will decrease your HP to 15 on the contact if you have more than 15. He will drop essence of destroy. With it, you need to craft the Demonic armor. You can also get the Annihilation. That is pretty good sword for the early game.

True Cultist
First of all, create a Lunar Cage and right-click on it. You'll meet the True Cultist.Recommended equipment: Aureum armor, Fierium sword and Blazing Glory. He can drop Constellation, sword which can teleport you for 12 blocks at day and 24 blocks during night. It is very helpful for next boss.

Reld
After Cultist, build the portal by Runes and activate it by Rune of Reld. Then kill a lot of Reld Golems and get Ancient Reld alloy. Then mine every ore from Reld, create equipment, prepare golden apples. After preparations, craft a Heart of Reld and use it on the Block of Reld Energy Crystal with Reldrock under it and rune above it. After this actions you'll summon Reldius. This Boss jumps very high and cause explosions on landing. Use Constellation or Ender Pearls to avoid it and damage him by double-barreled reld rifle.If you can't kill him, find enchanted golden apples and craft Astral Armor and terra javelins. He drops Idol of Steve and Crysalis. Save both of them and go to Vornol by the same portal.

Vornol
Then find a temple with the face of vornol. First quest requires gnikumus. This thing drops from guard of vornol with 5%. Other quests doesn't requires explanations. After quests, go back to Reld and grind golems for prismus. With prismus and ectoplasm from corrupted souls in nether create a phantoplasm. Use it on Vornolwood Log to summon next boss - Forester. Recommended equipment: passive dimensionite stuff and crysalis. He drops Prime Poleaxe which cause more damage if target is standing on wooden block. He also drops lifeblood. With it you need to craft livingsteel armor. It provides you big boost every 10 second by the cost of hunger.

Frigid and Duneland
Now you need to go into the Frigid. Use Fierium armor to create an aegis of blizzard. Find temple and summon the ice elemental. This boss is pretty easy. Use living steel or chill equipment. After this boss you have to go to the Duneland. Find a temple and summon the sand elemental. Use Adanium armor or armor of snowfall.

Purgatory
Next go to Purgatory. There you need to find Profaned desert biome and ascended desert biome. Grind every mob and create soul clusters. With it you can craft soulite ingots. With it you need craft equipment. In this dimension from Betrayed warrior you can get loarius.

Night Army
Next thing we will do is fighting Night Army. Create profaned heart and use it on the night totem to summon Night Army. You'll have 5 minutes to farm resources. With night bones and phantom ingots create phantom armor for ranger.

The Great Observer
Next boss for you is the Great Observer. Recommended equipment: absorber, yin yang, triad, shadowflame, soulite or phantom armor, constellation, soulite sword and pluck. Boss is pretty hard because of his 3 phases. In first fase he slow and weak, summon minions. In second he become fast and teleport when hit. Third fase is same as second, but he will break your armor very quickly and become stronger. If you can't kill him, craft overloaded beethroot, get totems of undying. He doesn't drop good equipment.

Reapers
Now in your world you can find new mobs, Reapers. With their drop you can craft next armor. With this armor you can focus almost like in hollow knight (press shift to focus). Next boss is the Supreme Reaper. He is pretty easy. With his drop you can make new ranger armor.

Arcanir
Now it is time to Arcanir. Grind mobs and create new equioment. Arcane armor provides you flight with cost of attack and mining. Next boss is the Beast of Arcane. Just avoid his attacks and hit him by melee weapon.

Brothers
After Arcanir you can fight Avatar of the Bealex and Avatar of the Lizekh. You can use Mysterium armor or Reaper armor. First of them should be Avatar of the Bealex, but it doesn't matter. With souls of Bealex and Lizekh you can create Vita ingots or Eclipse ingot. With that ingots you can craft armor.

Forge
Ok, now we are ready to go into the Forge. In this dimension you have 2 ways of progress. You can find brimstone caverns and create brimstone armor and brimflame or you can mine ancient debrees and create active dimensionite stuff. In the forge you can get meet evets. They will drop you lordius. Next boss will be Talos. He has a lot of attacks, so there isn't tactic for him, but I recommend you Active Dimensionite equipment, absorber, yin yang, triad, flare, mindbreaker, brimflame and constellation. He drops good stuff. With ichor you need to upgrade your staff, cataclysm and catastrophe are pretty strong, but I recommend only cataclysm.

Fraus
In the Fraus you can upgrade your equipment from the Arcanir. You can find some new mobs including Mimics! Boss of the Fraus is the Mimic king. He is not so easy, but still ok.

Ascended
Next boss will be ascended. Summon him by using Rune of Gods on the Night Totem. He has 3 phases. In the first he will summon evets, in second he become very fast and invincible to range weapon. At the third phase he become invincible and heals 25 hp per 5 second. You need to damage him as much as you can. After third fase he will back to second fase. Recommended equipment: radiant staff, cataclysm, brimflame, empire aegis.

Abyss
After Ascended you need to enter the abyss. In the abyss you need to mine Thrillium for the new equipment. Search for the Sword in Stone and summon new boss - Shade of Pain. He drops Charm of Abyss that will really help you.

New Homeworld
In the Abyss search for the beacons and enter it using Empire Aegis (or any product of it). Fight old bosses for pale ore and create a pale lense and enter the new homeworld. Search for the Pale Knights and create a new equipment. Craft the heart of Abyss and summon the Pale Guardian. Defeat him and upgrade your equipment.

The Prime
Create the First Rune and enter the Prime. Use boss summon items to summon elder bosses. Use the Face of Vita to summon Vita. You'll have 2 minutes to deafeat Vita. So, defeating Vita is the end of game. BUT! You can try your power by challenging someone at the other side of screen