Your grammar accepts "2-3-5" as:

       Answer
        Op=-
     /       \
   Mult      Answer
   Pow          Op=-
   Term       /      \
   Literal   Mult    Answer
   "2"       Pow     Mult
             Term    Pow
             Literal Term
             "3"     Literal
                     "5"
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So it's evaluating as 2 - (3 - 5), as the grammar allows. If perhaps you can write your grammar in a more left-recursive fashion, such as:

   <objrule: Answer>
        <X=Answer> <Op=([+-])> <Y=Mult>
      | <MATCH=Mult>
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This sort of transformation carried throughout the grammar might do the trick.

Disclaimer: I've not written a compiler, nor played around with grammars in quite a while, so take this with a grain of salt...

...roboticus