Pokemon Red/Blue is full of ways to develop strategies. I will now tell you the most important ones.
How to defeat a Mewtwo user in a link battle. In order to defeat a mewtwo you need a pokemon that can with stand at least one hit from mewtwo and that knows a sleep move.(Parasect, Exeggutor, ect.) A ghost pokemon most likely will die after a psychic attack) Well, you must then put the opponet mewtwo to sleep. Now, you must know mewtwo's weaknesses. It happens to be bug. So take out that pokemon and start attacking. The opponet will probaly switch the mewtwo so when he puts him back just switch your grass/bug pokemon back and you should be good off.
Your best bet as a pokemon with moves against a mewtwo is this"
Parasect
Slash(good bug attack)
Swords dance( doubles your next attack, very good when used against an asleep mewtwo)
Leech Life(Good Bug attack)
Spore(Will put to sleep majority of the time) ~--------->This is a compliment of Pesmerga
But I've noticed Mewtwos usually have blizzard and a Parasect can't survive one blizzard. So this is a good strategy.You must change the status of Mewtwo( Paralyze, Sleep, Confused) I suggest either Paralyze or Sleep. An Exeggutor with Sleep powder, Psychic,Double Edge,Mega Drain. This is even better than Parasect.
An Alakazam could also rip up a Mewtwo. It should have Thunder wave,Seismic toss,Psychic, and this last slot is optional. I'd suggest Reflect maybe. Use thunderwave to slow Mewtwo down and a cance of fully paralyzing it. Then Seismic Toss will take away an amount of damage equal to your level. So a level 100 Alakazam against a level 100 Mewtwo should need 4 (5 if Mewtwo has good stats) Seismic Tosses. Reflect is if you want to survive more hits from Mewtwo by raising your Defense.
Raising a Pokemon Unless you are using a certain method I am not allowed to tell, rare candies will weaken your pokemon. They are no good. They are a waste. Using them will give them half the stats they will normally get if you train them right. It is good to use no rare candies and to raise your pokemon by fighting and gaining exp. (experience) in the game. It is good to duplicate TM's when raising pokemon. Also, Use stat enhancers(carbos, iron, ect.) on your pokemon.