Before you even think about going for Platinum in ANY event, you must first get gold in all 53 and achieve the "Au" achievement. Once you have done so, a notification will appear saying you can now go for Platinum on all events; any event that has already earned a Pt score will be automatically upgraded. Go through all the events again, getting Platinum requirements.
To make some of the events easier (such as Raid and War), finish a playthrough and start a New Game + on Easy to finish them off.
For a list of Platinum requirement times for all events, please use this guide.
Warning: DO NOT get Pt scores on everything before unlocking the Au achievement. For some reason, it appears they do not stack and will glitch, only giving you Au and not Pt, despite all your events being at Pt status. The only way to fix this is to start a new game and do them all over. So, be sure to leave at least one event at a simple Au ranking to unlock the Au achievement and the ability to score platinum, then finish it off with a Pt score if you need. You can do up to 52 at Pt ranking before unlocking the Au achievement.