i think i wasn't really clear when i explained myself: i do have access to the move eraser. I just don't have anything to put on my mons. Where do you find psybeam ? U bought it ? Or u just catch mons until u find one with this attack ?
Yes, I bought it. I just don't remember if this was a tm selled in only one city or if it was in all with after some badges.
But you already have access to some powerful move, in olivine city iron head is great, in ecruteak there are fire/ice/thunder punch who're also very good, especially for that dragon gym with your physical ice mons.
You also have good move in the casino, in goldenrod city.
The really good moves will come in shop a bit later, but when you'll get those, the challenge will be a bit off until you reach the third part of the game.
I rolled on every kanto's gym leader with dugtrio earthquake(available in shop at a moment) and Alakazam psychic (which was given by a PNJ).