OK, so you've nearly completed the game. all you have to do is kill a monkey with a sledgehammer. Sound easy? Well, it's not THAT easy. There are many approaches, but the ones that seem most effective are:
*needlers (the do a lot of damage to brutes in general)
*dual brute plasma rifles (after you soften him up, dual brute plasma rifles DECIMATE him, which is surprising seeing as how they're wothless agains normal brutes)
*Plasma Sword (It deals some damage, but not nearly enough, and it brings you within reach of his hammer)
I personally finish him off with brute plasma rifles every single time. Also, don't rush in or meele him. you'll get killed. let him fight the elites, then hit him when his shield is down.