Can someone tell me how to beat sheegoth because I keep dying?

The key to beating Sheegoth is dodging him - try and stay as far away as possible from him for a start...
Now - your Power Beam is useless when it comes to damaging Sheegoth - so Missiles are your weapon of choice...
BUT - you can't just fire Missiles at him to kill him - you need to tire him out first...
So - fire charged up Power Beam shots at him to "power him up" and he'll shoot some Energy Balls at you - dodge them and then he'll start gasping for air...
Now's your chance - now he's vulnerable - this is when you let him have it - so lock on to him and send some Missiles into him - you'll now you've damaged him if he flashes red when the Missiles hit him...
You'll need to repeat this process a few times and hit him with about 10 Missiles to take him out to acquire your new Beam - so - good luck man =)

The key to beating Sheegoth is dodging him - try and stay as far away as possible from him for a start...
Now - your Power Beam is useless when it comes to damaging Sheegoth - so Missiles are your weapon of choice...
BUT - you can't just fire Missiles at him to kill him - you need to tire him out first...
So - fire charged up Power Beam shots at him to "power him up" and he'll shoot some Energy Balls at you - dodge them and then he'll start gasping for air...
Now's your chance - now he's vulnerable - this is when you let him have it - so lock on to him and send some Missiles into him - you'll now you've damaged him if he flashes red when the Missiles hit him...
You'll need to repeat this process a few times and hit him with about 10 Missiles to take him out to acquire your new Beam - so - good luck man =)

Start by firing your beam at the Sheegoth to charge it with energy. Then, while locked on, move away from the beast. This will trigger it to use it's stored energy and fire energy balls at you. After each session of firing energy balls, it will hyperventilate, giving you the perfect chance to fire Missles at it. If you run out of Missles, use Bombs. Repeat this to defeat it. Your reward is the Wave Beam.

If you mean the big one, the Plasma Beam is the best weapon, but Missiles are rather effective.
The Plasma Beam can damage any part you hit. Missiles can only damage the head/mouth area.