How do you get the solaris pistol on Metal Gear Solid 4

The weapon to which you are referring is the Solar Gun. To obtain this item in Metal Gear Solid 4: Guns of the Patriots, you must defeat The Frogs (Haven Troopers) at Advent Palace non-lethally, as well as the B&B Corps bosses when in their Beast forms. After each battle, you will find a doll (five total), which you must collect. You can keep track of which dolls you have in the galley of the Nomad during mission briefings. You do not need to collect them all in one playthrough as long as you are using the same game save.