Leveling Guide
Disclaimer: If you are reading this guide and your character is below level 180, you should create another character and participate with the "Jump Start". After character creation, you will be given the option to start as a Level 180 (although it says Lv. 185 in the UI) or a Level 1. Choose the Level 180 for a head start and some extra items as well. This is a part of the game.
All characters created now start at Level 180 by default.
At the start of the game you have a bunch of free boxes in your inventory. Make sure to open and equip the "Elite Champion Costume" box and the "500% Strengthened Veteran 7 days" box. This gives you massive EXP bonuses at the start.
180 - 210+ Arcadia Parties (Highly Recommended):
Occassionally, there will be people looking for members in Arcadia map. Their shouts in match chat will look like this: 
These are parties at arcadia where you can AFK to level really quickly. If you are just starting, you should have your holy water with you and you can warp to your party members. Do not attack. You are there just to leech experience and in turn, having you there will increase the members in the party thus increasing the EXP for the people killing the monsters. This is the fastest way to get to level 200-210 at the start. I recommend getting at least 210 so you can have your Grand Meister 2 skill ready and maxed.
180 - 195 Mitera Main Quest (Best jump start):
At the start of your leveling, it is recommended to finish the Main Quests in Mitera. These have tags of [Main] in the Quest tab. These quests start at Debanni. Use your Holy Water to warp to him. You will be tasked to kill a bunch of monsters which you can very easily do at the start due to the free weapons and other accessories. Once you are done, you should be around ~Level 195. This should not take more than 1 hour.
Disclaimer:
"Scout Lanerin" NPC is the "Ranelin (Guard)" NPC in-game.
180 - 200 Mitera (AFK):
Farming mitera fragments and gniar scrolls is one of the things you will be doing at the start. Since you are bound to farm them anyways, you can afk straight away at the Mitera Map instead for the early levels. This is a killing two birds with one stone situation.
180+ Mitera's Red Hole Event (AFK):
Every a couple hours, the Red Hole event starts at Mitera. Note that there is no specific time for them to start. This event is triggered by the number of monsters killed in the map. Once it reaches a certain point, the event will start. Watch out for it closely. Once the event starts, warp to any of the three Red Holes and look for parties. The more people in the party, the better the EXP. There will be a lot of infectees that will spawn which gives really good exp at the start especially if you have a lot of people in your party and you utilize the bonus EXP boosts.
200 - 230Arcadia Prison (AFK):
This map is the absolute best when it comes to AFK leveling. However, you will be needing Arcadia Protection and decent gears to deal damage. Note that even though you can survive with just the free gears, other players will not like it when you deal very little damage compared to them as you will be slowing them down.
210 - 230 Active Dungeoneering (not too viable):
As of January 2023, actively farming in dungeons still offer the best EXP in game. However, this is subject to availability and the willingness of players to do it with you and the gears you have available. Kastrioti Mansion -> Tower of Magic -> Qune Temple are the best party EXP in the game. Another problem here is that people tend to solo Kastrioti Mansion with alts so that dg is out. Tower of Magic and Qune Temple are difficult for newer players. Also, Qune Temple requires reworks for better drops for Qune Earring materials. Once this happens, actively leveling especially for geared level 220+ players in Qune Temple will be viable.
Massive thanks to Nidu and SmalleR. None of the guides would have been done as quick without them.