As per request from some friends, I will be making my own post on this since I had posted the extent of using Ein Lacher to level up on someone else's discussion.
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(this 1st section applies only if you are under lvl 80 , 2nd part will show how to get from 80-90)
Basically what you need is to do 10 runs to get 10 honor medals

So you can purchase the Exp Potion from the Ein Lacher Shop at either the Cohlen Docks Recruit,Rocheste or Malina Parrot

Once you have acquired the potion .you will want to choose a season 1 battle that has the highest base exp and you want to do that raid/battle 10 times. I chose Betrayal to showcase this as it has one of the highest at 50k+.
Below are the pictures of each run with the folllowing buffs; 150% Exp Booster(from current level up event),VIP,Guild Bonus
1st run

2nd run

3rd run

4th run

5th run

6th run

7th run

8th run

9th run

10th run

Even if you do not gold run any of the 10 runs , the penalty I have seen from purposely dying is at most 7k-8k EXP nothing more.
So you would this until you can not earn anymore honor medals.
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Congratulations! If you are reading hopefully you have made it to 80+

. Welcome to Part 2 +fight
now from here the exp potion is irrelevant, as you will be doing 10 runs of season 2 now instead of season 1 and the exp potion does not work on season 2. Nonetheless, doing 10 runs of Season 2 RAIDS I repeat RAIDS gives approximately 10 Million more Exp than 10 runs of Exp Potion Runs with a 50k+Base Exp Season 1 Battle/raid.
below is a pic from an Ulchas with some deaths/hits to show how little it actually affects the exp, with the same buffs as above
150% Exp Booster+VIP+Guild EXP

and with this I wish the new and existing mercenaries goodluck in leveling to 90!!