This is the song in Level 20: Gotcha! and 26 The Abondoned Mines.
G|---------------------------------------------------------------------------|
D|---------------------------------------------------------------------------|
A|---------------------------------------------------------------------------|
E|--0-3-3-3-3-4--------------------------------------------------------------|

then take it up a string

G|---------------------------------------------------------------------------|
D|---------------------------------------------------------------------------|
A|--0-3-3-3-3-4-------------------------------------------------------------|
E|---------------------------------------------------------------------------|

then back down

G|---------------------------------------------------------------------------|
D|---------------------------------------------------------------------------|
A|---------------------------------------------------------------------------|
E|--0-3-3-3-3-4--------------------------------------------------------------|


then up

G|---------------------------------------------------------------------------|
D|---------------------------------------------------------------------------|
A|--0-3-3-3-3-4-------------------------------------------------------------|
E|---------------------------------------------------------------------------|


then up one more string

G|---------------------------------------------------------------------------|
D|--0-3-3-3-3-4-------------------------------------------------------------|
A|---------------------------------------------------------------------------|
E|---------------------------------------------------------------------------|

then back down and down again.
thats the bassline its very simple.