Want to beat a Mega Man game? Or defeat robot masters here with their weaknesses? This post is here to help. I shall start with the first game and it's remake.

Mega Man 1
Guts Man can defeat Cut Man with the Super Arm.
However, pathetically, he can be taken down easily with the Mega Buster, which helps out more when facing him first or fighting him in Wily Castle.
The rolling cutter can beat Elec Man. BTW, did you know that this grid also helps with Mega Man Powered Up's Old Style Mode? Oh, and be careful... this tip on Copy Robot is also a pain in YOUR side, too...
Yep. Thunder Beam can take YOU down as well. Which is bad... unless you do this on Copy Robot.
Elec Man can beat Ice Man with the Thunder Beam, too.
Fire Man is weak to Ice Slasher. However, as our fellow user, Mikey76500 said...
Mega Man is also weak to Ice Slasher.
And Bomb Man is weak to Fire Storm. What? Why is it shown as Fire Chaser? Well... that's basically what it is in the original game. Kinda.
Bomb Man can beat Guts Man with the Hyper Bomb. Which, of course, starts the cycle all over again.

Sadly, Mega Man Powered Up is different on New Style Mode...

As always, it's Super Arm Cut Man in the face. Not sure if it's the same with the Mega Buster thing, though... maybe someone can test it out?
However, with Oil Man and Time Man added to the mix, most weaknesses are switched. For instance, you now can beat Bomb Man with the Rolling Cutter.
And our winter soldier goes down because of the Hyper Bomb.
Fire Man and Cut Man are the only ones to keep their weaknesses... Like Cut Man and Wood Man are secret bosses in a certain game... shh... no telling! While in secrets, Cut Man is also in Mega Man X8.
And the new guys come in here. Oil Man is weak to the Fire Storm... which still looks kinda like Fire Chaser.
Elec Man is now weak to Oil Slider. However...
You're still weak to Thunder Beam. Luckily for you, that's not true! ...Only for Mega Man and Time Man...
...Yeah... That...
Guts Man really wants to be done on time... which might explain this weakness.

This guide will come in handy for Mega Man 1 and it's remake, Mega Man Powered Up. ...Oh yeah, and the MM1 section of the Wily Wars for the Genesis...
Time's up! We shall see you later for a guide on Mega Man 2.
For more information on the RPG weaknesses, check the database! Because this is manly for the normal games.