The following are known Action Replay Codes for Advance Guardian Heroes on Nintendo Game Boy Advance (GBA).
These codes were also known as GameShark Codes and in most cases will work on both devices.

These codes are for the U.S. version of the game.

Master Code – Must Be On
f9479bc5
df2b6563
08301b72
d4c61322

Press L + R for Max EXP
e181fd7d
760faaeb
a4459693
d56532cf
00cbf1f0
1e97ceb7

Max HP
9795397e
b74593f2

Infinite HP
27dec666
a0e23c53

Max MP
a31efd0c
c25cb34a

Infinite MP
6a350e7d
e16fb9e3

Max/Infinite Rage
6c002304
82efe589

L = More Devil Time
383c3f29
c175e6fd
409f1a77
b91aa631