Nec illud proelium fuit: male sustinenti arma gladium in iugulo defigit, iacentem spoliat. That battle wasn’t: he thrusts the gladius into his throat, he despoils him as he lay.
I’m not sure how male sustinenti arma is supposed to work.
Both here and in your preceding question, a dative of reference is used as a kind of possessive. This is particularly common in Latin, instead of a genitive, where the object refers to an inalienable possession such as a body part.
Here male sustinenti arma, “hardly able to hold up his equipment,” refers to the unfortunate person into whose throat the sword is thrust. In the preceding question, feroci iuveni refers to the person whose spirit is moved.
“X thrust the sword in his throat, as he was barely able to hold up his equipment.”
“His sister’s wailing moves the spirit of the savage youth . . .”
As Adrianus suggests, movet animum may mean “raises anger” depending on the context, but I would translate “raises the anger of the savage youth,” i.e., as an English possessive.
Nec illud proelium fuit – “that wasn’t a real battle,” i.e., the fighters were so unequal that it couldn’t be called a real battle.