Please make an effort to post to the right forum - because this is a question regarding the classic editor I've moved it to Newbie Central.
A combat system using the classic editor would be a lot of work for little gain. "Dice rolling" can only be done in the advanced editor, unless you're counting on your reader to have a die handy and be honest with their results. There are a few posts on making a combat system in the Advanced Editor forum; I suggest you learn how to operate variables and scripting and then go from there.
If you wish to continue using the classic editor (just a heads up, the advanced editor is way better regardless of if you're using its advanced features or not) then here's how you should make a combat system: the exact same way you make a regular storygame. Provide a number of choices for your reader to make, and then write what happens when they click on those choices. Easy.