In my experience there is no set format. I have heard some rubbish ones from officers and some even worse from solicitors. The key is to put in the effort as if it were you standing there under threat of detention and other life altering experiences.
Deal with his personal background - paint a picture of someone that is accurate and shows what sort of person he is, what sort of soldier he is and could be, but don't be shy about playing up any under-privileged tough upbringing.
Deal with the incident itself - why was the soldier drunk, why did he hit the RP Cpl etc?
Finish with an explanation of how he feels now - sorry, ashamed, scared, repentant.
Type the whole thing up in perfect English, double spaced and copied for each member of the Court. Hand it in.
When the Court asks the soldier if he has anything to say make damn sure that he uses the chance to back up your plea by saying sorry etc. Rehearse this in advance.
As far as I'm aware (only been in court 4 times myself) a plea of mitigation should REALLY attempt to outline any mitigating circumstances that might cause a judge/court to look more sympathetically on your reasons for having committed an offence. ie: I was speeding because my wife was giving birth in the back seat and I was trying to get to the A&E.
However, I have twice received significantly reduced punishments (from Chelmsford and Carmarthen Magistrates Courts respectively) when I gave them long and detailed pleas of mitigation that had nothing to do with my reasons for committing the offence, but everything to do with the impact of the potential punishment. They paid not a blind bit of attention until I mentioned that if I was convicted and lost my driving licence, I would be subject to further military disciplinary action. This 'double jeopardy' effectively led to a much reduced sentence from the courts.
Hope it's of some help. Of course, if you're trying to put in a plea of mitigation in an attempt to get an LS&GC awarded, forget it! LOL