Super Smash Bros. Melee

Yes. If you win, you get a trophy. Giga Bowser.

I don't think there is a "strategy" to use... Try to pay attention to its moves and dodge them accordingly, not being on close-range all the time.
Mario does have that fireball.

To get the Giga Bowser trophy you need to beat Adventure mode on normal or higher difficulty under 18 minutes without continuing (losing all your lifes). I highly recommend you to do it on normal and with the maximum of lifes you can have.

For the adventure levels I recommend you to ignore all the enemies and try to get to the to the finish as fast as possible, also when you are fighting the Kirby team try not to defeat them under 30 seconds 'cause that will make the giant Kirby to appear wasting time and probably lifes.

Once you get to Giga Bowser (after defeating Bowser) try not to get hit much 'cause it can K.O. you easily, especially 'cause its attacks have a big range, also remember that you might have to cause it like 300% - 400% so he gets K.O.'d.

If you do it right you should see its trophy going into the black, if you for some reason did something wrong, the trophy will smash and break into pieces meaning you didn't get the trophy, so try to beat the mode as fast as you can.

After you beat giga bowser, the trophy is suppose to break into peices. If you get the adventure clear bonus after you beat regular bowser, then you were either too slow or you used a continue. If you don't get the adventure clear bonus, then that means you still have to fight giga bowser. And I highly recommend that you have at least two lifes when you battle giga bowser if you are not a pro.

What I had to do was just use Yoshi. I did a single jump, down A (the constant kicking), jump again and then use down A again (I sometimes was able to do it two times before I reached the ground again). Don't try to grab him, remember: he's too big!
Sometimes it help to press the direction of the opposite side of the stage when jumping (so after you get him to the right side, try doing your second jump to the left so you can get a good landing before you start the damage again)

Just punch him until you have one life, then do a smash attack. He'd have about 450+% damage, which is more than enough.

And, yes, you do get a trophy.