"One last trick in the bag" is like "One last trick up my sleeve," saying you have one more, er, trick (move/ thing/ attack).
"He's almost got me beaten, but I've got one last trick in the bag."
[I've got one more ultimate move that will hopefully make me victorious.]
That one's hard for me to explain (sorry!). . . but I've only really heard it in action movies anyway.
- For Teachers