The end isn't hard to beat if you know these secrets
1 Type in the Konami code(Up Up Down Down Left Right Left Right Square Triangle) in the menu on the map screen to see the location of The End during your battle with him.
2 You can use the thermo gogs and the directional mic to locate him
3 After caves and after the river with the guards on the hovercrafts, enter the next area and a cutscene will begin. As soon as the cutscene ends pull out your SVD sniper rifle and aim it at The End in his wheelchair. Before he gets taken away, shoot the guard in the head, then shoot The End in the head a few times. If done corectly The End will say something and explode. Later in the game when you get to the jungle where the fight was supposed to be, you wont have to fight him and the jungle will be filled with guards from the Ocelot unit.
4 When you enter the fighting area for the first time, save your game and quit. Go into the clock settings of your PS2 hardware and set the clock forward about 6+ hours. Load up the game and you will find that The End has died from old age!!
And to answer your Solidus ? He's looks to be older than snake and liquid. He has a gray beard, grey hair and can absorb metal to become slightly bigger and SOLID. He later loses an eye to resemble BigBoss in a way.