Remember that the platinum requirement here is just to win with all five lives remaining. Therefore, you can pretty much just take your time to make sure you are always prepared to face the golem.

The best way to kill it is to use the secondary function (3 rocket fire) on either the rocket launcher or the homing rocket launcher. Other good things to remember are health locations, rocket locations of course, and the body armor location.

Try to spawn somewhere near the main multistory building. Inside on the first floor, pick up the rocket launcher. Then go up to the second floor and up to the homing rocket launcher. This will give you enough ammo for two kills assuming you connect with your rockets. After the first two kills, look at your health and get more if you need it.

Head back toward where you got your rockets and pick them up again after they respawn (it takes 30 seconds for a weapon to respawn). You will now have enough ammo for the remaining three kills. Just be careful about your health.

Another thing to make a note of is what weapon the golem is using. It tends to go for either the grenade launcher or the shotgun (x1 or x2). If it has the grenade launcher, make sure to dodge the grenades and be on the lookout for fire grenades as well. If you catch on fire, there's pretty much nothing you can do about it. The preferred weapon for the golem to have is the shotgun (x1). He'll probably hit you once or twice, but it shouldn't hurt too bad. If it has the shotgun(x2) you need to be a bit more careful, and only approach it if you have a lot of health.

I guess the main thing to remember is that you are not timed on this at all, so do whatever you have to do to make sure you are well prepared for each encounter.