Preparation before leveling up to 80+

Reach level 66, then immediately reincarnate at level 31 (unless you need reincarnation 100 to get the 5% bonuses).
Before leveling up to 65+, if necessary, get a potion that offers +800 magic defense points by completing the Oceans Company reputation to 16k. Turn in 1200 "ancestral ornaments" in the repeatable quests between levels 63 and 65.
Get power-leveled from level 66 to 80 if possible. Make minimal investment in specializations that offer offensive stats (attack, critical damage, critical rate, penetration) and defensive stats (defense, critical rate reduction, sufficient movement).
Acquire level 3 or 4 talents that offer significant offensive stats or a very powerful combination of talents. It is unnecessary to complete all talents.
Before leveling up to 80+, if you are not a mech or traveler class, you must get 50k reputation with Kaslow, Ilya, and Techno, as well as 25k of each gathering reputation from the 3 towns (e.g., Miners' Syndicate), 16k with Elsa, and at least 16k at the Frost Altar.
If you are a mech or traveler class, your level 1 performance stones are available at the altars on different maps.
If you do not want to get all the level 1 performance stones, you will need at least 16k reputation with Techno to access the +400 physical defense potion.
For Elsa reputation farming (levels 78 to 80 for repeatable quests), you need at least 16k to access the physical reduction potion (-4% physical damage taken) and 25k for the magic reduction potion (-4% magic damage taken).

Leveling up to 80+: Basic Concepts

Mob grinding from level 80 to 90 is generally done on the 4D maps Ancient Polar Realm (north or south of the map) or Moonlight Forest (near the scorpions or cats). Test spots to find where you are most comfortable. Transmuted dungeons (TPT or Laby T) can be an option, but I do not recommend them.
After reincarnation, levels 86 to 90 are the longest part of XP farming. Each level takes much longer than levels closer to 100 because starting at level 94, you will have access to the Transmuted KEFT dungeon, which gives twice as much XP per mob as the 4D mobs.
There are quest lines that must be done as they give access to dungeons and important titles to farm. Check the full list in the GF guide for details.
To level up quickly to 80+, alternate between 4D mob grinding (duo is very profitable) and quest lines on Temporal Rift maps, which give a lot of XP since a recent update. Details on the maps to visit and the method to follow will be provided in the GF guide.
Sometimes, an event called "Hunting Event" available from an NPC in Kaslow can cost 3k gold, but the mobs in this level 100 zone give four times more XP than the 4D mobs. Check the patch notes to see if the event is available and refer to the solution in the GF guide for more details.

Complete Guide to Leveling Up to 80+: Mob Grinding / Quests to Do

Levels 66 to 82:

You have access to the Kaslow in Flame Dungeon quest line starting at level 66 with "Desperate Call for Help." Follow the KEF quest guide until the quest "Advance on the Front Line 3."
Starting from the quest "Very Suspicious 1" available from "Butler Tomas in the Kaslow Desert Area," you will access the "Rebel Base" for KEF farming starting at level 86 and Transmuted KEF from level 94.
To reach level 82, do 4D mob grinding (duo is recommended). You will need enough movement, 9-10k physical defense, more than 150 evasion, and at least 20-25k attack, 100 critical rate, and 300 critical damage.

Levels 82 to 85:

In the Kaslow Desert Area, you will find a passage to the map "Peninsula of Sorrow." Complete the main quests to access the maps "Glass Mountain" and "Plateau of Hope."
Stop at Glass Mountain and switch to 4D mob grinding to reach level 85. You can also do profitable repeatable quests and Daily Quests (DQ) or take advantage of the Hunting Event if available.

Levels 86 to 88:

Resume KEF quests with Tomas in the Kaslow Desert Area and complete the quest line in Rebel Base. You will also have access to the Kaslow in Flame Dungeon to get the "KEF Tome."
Continue the main quests in "Glass Mountain" and "Plateau of Hope" until level 87. At this level, start the quests in the 91+ zones based on your progress.

Levels 88 to 90:

Start the "Condemned Island" quests to obtain important titles. You can also buy the titles from the Black Market Vendor to save time.
From level 88, you can advance in the quests on the "Plateau of Hope" and "Smoking Basin" maps. Be sure to complete the zone quests up to level 91 to unlock access to the Distorted Dimension dungeon (DD).

Levels 91 to 94:

Complete all quests on the last trio of maps: Smoking Basin, Valley of Light, and Steep Port. Mob grinding is generally not necessary if you follow the quests correctly.

Levels 94 to 100:

From level 94, focus on the KEFT dungeon at Rebel Base. Make sure you have enough defensive stats to manage the rushes and optimize your equipment for efficient grinding.
From level 96, you can return to Steep Port to unlock quests up to level 100.