I'm not exactly sure how you have things set up, but I'll assume you have 2 text boxes, one for the X coordinate and one for the Y coordinate. If it's just a single text box you can parse out the individual X and Y by using the system expressions
tokenat(src, index, separator) &
tokencount(src, separator) system expressions.
Since you have sprites to represent each grid space you could give those sprites 2 instance variables to represent their X and Y grid positions.
Now you can just pick the sprite where the coordinates from the text box match the coordinates of the sprite. Then you can use the X, Y layout coordinates of the sprite so you know where to perform the strike.
There's other more mathematical ways you could accomplish the same thing, but since you already stated you have individual sprites to represent the grid spaces, this should work.
I hope that helps.