This Achievement is for getting to level 200.
It may seem daunting at first but it isnt so difficult when you use Grade in new Game+ to stack 2x and 3x EXP for a total of 6x EXP. Once you are able to buy both 2x and 3x EXP boosts beat the game entirely including all side quests and the Bonus dungeon to get to level 200. I hit level 100 before defeating the final boss(I did ALL side quests before the final boss).
Complete the quest "Striding Beyond the End" after completing the game. This will net you a lot of EXP with the 6x Multiplier.
The rest of the levelling will need to be done via the Bonus Dungeon. To do this you will need:
Dragon Slayer: +30% EXP from Dragons Enhancement Bonus
Feldspar Belt +20% EXP from Dragons Enhancement Bonus
Topaz Garment +20% EXP from Beasts Enhancement Bonus
Lindworm Ring\Anamalia Ring +40% EXP from Dragons\+40% EXP from Beasts Enhancement Bonus
Adamantine Boots Not required but I found that I typically always got a pair that had Three +18% Capability vs Dragons Random skills andMaster skill has a 6% chance not to lose a soul for a break soul.
Once you got all these pieces MAKE SURE YOU ENHANCE THEM until the skills mentioned are unlocked. Either ring will work for this. Anamalia ring is easier to upgrade.
So basically what you want to do is enter the Bonus dungeon and teleport to the First Stage. Set your party to defense only as you need to land the killing blow to get the bonus EXP. Go to the Boss which is the Green Lizard and Kill it. It is a Dragon\Beast type enemy which is why we want the EXP bonuses. Upon defeat you will get 1,000,000 EXP which will Level you up 2 level majority of the time. Sometimes you will finish just under 2 Levels but not often. Teleport out of the Bonus dungeon and repeat. The achievement will unlock when ONE character hits 200.