How to get the Gundam RX-78:
     Beat  the  game  on any difficulty with the Z Gundam. Then choose
the  Sazabi  and  play  until you face Gundam RX-78, beat him and then
beat  teh  game  and he'll be selectable in VS. Mode. (You may have to
fight many times with the Sazabi to get him to show up.)

How to get the Ball:
     Beat  the  game  on  any Difficulty with all 12 Mobile Suits. The
Ball will be selectable in VS. Mode.

How to get the U-Gundam (New Gundam):
     After  getting  the  Ball,  fight  as  Zaku II on any difficulty.
U-Gundam may show up and fight you midway in Story Mode. Beat him then
beat  the game. U-Gundam will be selectable in VS. Mode. (You may have
to fight many times with Zaku II to get him to show up.)

How to get the Zaku's (Char's Zaku):
     At the main menu (Story, VS., Memory Card, Option), press L1, L1,
L1,  L1,  R1, R1, R1, R1, L1, R1, L1, R1, L1, R1, L1, R1. A sound will
confirm correct entry. Go into VS. Mode and keep pressing Right

How to get the bosses Bygzam, Neue Ziel, Psycho, and Hydra:
Beat the game on Easy difficulty to obtain the BYGZAM
Beat the game on Normal difficulty to obtain the NEUE ZIEL
Beat the game on Hard difficulty to obtain the PSYCHO
Beat the game on Hard difficulty without any continues to obtain the HYDRA
(Note: If you obtain Hydra, all Bosses will automatically be selectable.) 

How to see the Full Ending:
     Beat  the game on any difficulty with the Wing Gundam. Heero will
comment "Mission Complete" and a special sequence will begin after the
normal ending.

How to get The Ball:
     The  real and sure way to get this gundam is to finish story mode
with  each  gundam twice, then in V.S mode, char gundam Zaku should be
selectable.

How to get Big Zam:
     To  obtain  big  zam,  finish the story mode on easy or medium or
hard (can use continues)

How to get Neviel Zeil:
     Finish story mode normal or hard (can use continues)

How to get Psycho mk2:
     Finish story mode on hard (can use continues)

How to get Hydra Gundam:
     Finish story mode on hard (cant use continues)

How to get Gundam:
     Finish  story  mode  with  the  Z-Gundam,  then choose any gundam
besides Z-gundam and Acguy, repeat this and Gundam will soon apear.

How to get V-Gundam or U-Gundam:
     The  names on the some copys are diffrent. Finish story mode with
all  the  with at least half of the gundams, U-V-Gundam will soon show
up, beet him and hes yours in V.S mode. (note getting this gundam will
erase all of the boss gundams besides Big Zam and Neviel, so u have to
earn them back)

How to beat Psycho mk 2:
     An  easy  way  to beet him, after using all your amo and specials
would  be, try to have less damage then him, then run around him so he
cant hit u till the time is up.

How to beat Hydra:
     Just kick his butt, hes not nearly as cheap as Psycho.

How to beat Big Zam:
     Just  use  the  fly mode down right then fly button and batte him
into a pulp.