I loved this game when I was 8 years old, there were no arcadeprehacks that time so I have to use my own game skills to win it. With much effort, I won the last boss when I was level20...got bored with the game and I left it at level 26

so heres it:
level1 - level3:
swift like wind, you can beat the human portal

level4 - level5:
using the little mission, get some money and buy some weapons

level6- level 10
as you get money with the little mission buy the best weapons and you can, beat it, and when getting skills, get more healing skills than attacking skills

level11-level15:
if you got through that(human portal) easily, you can go to monster portal...if you have enough hp and healing skills you can get through this easily, beat the whole monster portal to read the last walkthrough:

impossible portal:
first boss:
you got more to beat him more than heal yourself, prepare more striking weapons

second boss:
you would need the strongest physical weapon you need, you can get that by earning the money through doing the small mission again and again.

third boss:
if you beat the first and second boss, this boss is rubbish, kill him through physical and healing

last boss:
You must have to learn the shadow skill, which duplicate yourself. Use the most physical weapon you can find and equip more mana potions. you would need to attack the boss with the shadow because this boss recovers really fast, if he kills your shadow quickly make one, if you are strong, you can beat it with a few shadows. At first when i dont know to use the shadow skill i played forever with this boss healing and attacking him at the same time when my physical attack is so small.

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thats about it, feel free to ask any questions