To get real good tachikoma weapons. Look around the stage for a golden robot. Destroy it and when you beat the mission. You'll recieve a good tachikoma weapon. Good luck!
If you want to play as Aramaki, go to multiplayer and it allows you to select him.
You can pick up weapons that the enemies drop. All you have to do is finish the level while still holding it. When you start a new level the weapon will be in your inventory.
Just beat the game with any character in order to unlock a SpyHunter like minigame featuring Tachikoma, and a Movie Viewer that let's you watch all of the game's cutscenes again.
If you start a mission with only a handgun equipped, at the end of the mission you will be rewarded with that same handgun in its dual form - 2 of the same gun that can be equipped at once, only needing 1 weapon slot to hold.
You can use this method to unlock a dual version of each of the standard handguns in the game.
You must complete every mission in the single player mode with on of the characters. There is a weapon for every character in the game. All the weapons you unlock can be then used by the other characters as well.