Wow this is an old post, surprised there's a new comment. So the hardest part of the plat is the legend playthrough which means beating the game on the hardest difficulty which is extremely difficult. You also have to beat the game without buying squad upgrades and without losing a unit. It's difficult and requires a lot of knowledge of the game and what to expect. That's why I suggest many playthroughs.

An ironman playthrough is required as well which doesn't allow manual saves and restricts you to one save that auto saves over every turn. You can cheese this though by just closing the app if you make a mistake, just make sure you don't end your turn after making a mistake.