As with many other guides, this guide is intended to answer all of the "Where do I train" questions that we receive so often here.
A lot of guides neglect (what I believe to be) important aspects that can greatly improve progression rate. They also use a bunch of acronyms that new players simply won't understand, which lead to more questions. Hopefully, this can resolve all of those questions in one location.
Section 1: Training Areas
Note: I have picked the most easily accessible training locations(Typically Maple Guide). If not accessible through Maple Guide - use the World Map and search for the monster name
Level 1-10: These levels are usually covered by your tutorial quests. Follow the short quest lines and hit level 10 and get your Job advancement.
Level 10-15: Proceed through quest line OR kill some monsters in a nearby area.
Level 15-30: Move over to Flaming Mixed Golems.
In the Maple Guide, you can click the right arrow(from Recommended Quests) and switch to the next bullet point. This teleport will take you to a map nearby Ellinia where you can then use the World Map to navigate to Ellinia. Use Quick Move to access the Taxi and go to Henesys. From Henesys, use the world map and search for Flaming Mixed Golems. Go there.
Lv30-50: Gold Beach. All the maps here are good. Depending on the class you are playing, some are better than others. I typically prefer one map from the main Gold Beach entrance. To get to Gold Beach from Flaming Mixed Golems, you can go back to Henesys. Use the world map and set the Six Path Crossways(Center of Victoria Island) as your navigation point. On this map, there is a pilot guy about half way up the tree(Stay on this map). Click on him to get to Gold Beach.
Level 50-60: Riena Strait. At level 50 you gain a warp to Riena Strait through the Maple Guide. Just follow this and do the quests here. The quests are actually slower than grinding at the rock slab monsters towards the end of the strait. But just do the entire quest line anyway as you will get a face accessory. Alternatively, use the Maple Guide to teleport you to Wild Boars at lv55 and stay there until level 60.
Level 60-80: Copper Drakes in Sleepywood. To get here, navigate to Perion(If you are at Wild Boars) or use any of the previous Maple Guide warps to get you back to Victoria island. Go to a main city and take the taxi to Sleepywood. Go one map to the right and train here
Level 80-85/90: White Pangs: Use the maple guide to get here
Level 85/90 - 100: Sand Rats at Sahel 2: Use the Maple Guide to get here(only available at level 87 I believe, but you can start training here efficiently at around 85). You can use the Maple Guide to teleport you to the Pyramid. From there, you can navigate to Sahel 2 with the World Map.
Level 100-104: Zakum. All classes can solo Zakum at level 100. It takes two runs on Non-burning non-2x EXP days to get to Lv104 from Level 100. So, go here for those 4 levels and hopefully get face/eye accessories.
Level 104-113: Runts. Use Maple Guide to get to the little yellow furballs(Jub Jubs). Train here.
Level 113-130: Robos. Teleport to Robos using Maple Guide
Level 130-140: Blue Dragon Turtles: Teleport here using Maple Guide.
Level 140-150: Dark Cornians: Teleport here using Maple Guide
Level 140-160: Mysterious Path 3: This is the map name, the monsters there are like Selkie and something else.
Level 130-160: Moon Bunnies: I don't recommend going here if you are a new player as they do not drop Level 130 or Level 140 equipment
Level 150 + : At this point, you don't really NEED to grind anymore, there's a check list of stuff that you need to do which I will cover.
Section 2: StarForce/Potentials
I was debating putting this above the training section, but whatever
In your item inventory, there is a Magnifying Glass and also an anvil looking icon in the bottom right of the window. Items that have Potential slots available require the Magnifying Glass to be used. You typically want % stat on the potentials.
Tiers: Rare -> Epic -> Unique -> Legendary
Be sure to use all Runes of Riches and aim specifically for the equipment. It's very possible to get an entire set of Epic potential equipment with main stat before level 140.
Starforce equipment whenever you want before, as it is cheap(takes about 5 minutes) It's not too important for training. You want to make sure your weapon is 5 starred throughout training though. Also when you get Epic potential equipment, you want to discard whatever you have and transfer them up(using Hammer icon). So that you can carry that Epic Potential all the way until your Level 140 gear(Saves ALOT of mesos and time in the future).
IMPORTANT: Focus on Cubing(Look at other guides for this if you don't know what cubing is) your Secondary -> Emblem -> Badge. In that order(Badge should be Ghost Ship Badge)
Section 3: Level 150+? Ideally by the time you reach this point, you should have full Pensalir equipment(with whatever potentials, but Epic + mainstat would be great). Most classes have around 50-70k range at this point.
Go get your Ghost Ship Badge(obtainable from Boat Quay Town from Ulu's Quest)
Do Horntail Prerequisite(Very fast, just accept the quest and it will teleport you there)
Do Gollux Prerequisite quests(You need to do CrimsonHeart questline and switch to Tynerum once it becomes available). Found in the lightbulb on your left
Do Yu Garden quest line(Gives you access to a Black Market merchant in which you do quests on a board to get Yu Garden coins). You can buy Epic Potential Scrolls for 30 coins, and you can get 26 coins per day from the board
Finally, just do the following every day until you hate yourself:
Zakum, Horntail, Hilla, Magnus, Gollux, Ranmaru in any order you want at the hardest difficulty you can(Probably normal Zakum, easy/normal Horntail, Normal Hilla, Easy Magnus, Easy Gollux, normal Ranmaru)
Beyond this, there are plenty of other resources. You might want to get Commerci, and some other random stuff done at some point.
Will add more at some later time, but this is just for all the new people asking where to train.
There have been a lot of comments regarding my comment about not needing to grind at 150+.
The purpose of this statement is that efficiency drops significantly. Why? Because you're better served by increasing your range at this point. At around 150, you unlock so many ways to increase your range that you should do that before seriously considering grinding further. The difference between a 2HKO and OHKO is a world of difference in training time. And it's even greater when you're weaker. Presumably anyone recommending that you grind on Road to Oblivion 4 at 160 is ignoring the fact that any fresh character at that level will have around 100k~ range(with suboptimal potentialling) which makes training bad.
Prioritize the simple equipment gains(probably only takes a few days). Even leveling a link skill is more productive than training with such a low range.