Syndicate

As already stated by https://www.trueachievements.com/NinjaKIUK.htm, you have to complete the following levels on hard difficulty (3 stars):
1, 2, 3, 4, 5, 7,8, 10, 11, 13,14,15, 17, 18, 19
(basically the whole game without the last mission which consists of the difficult final boss fight)

I thought some parts of the game were tough, but all in all I rushed trough pretty quickly with ease. Some boss fights are hard and you can get trouble in open areas. My tips for Syndicate on Hard:

Guns
Use the right guns! I used mainly the CQC-11 shotgun, the Kusanagi assault rifle and the TAR (because there was no ammo for my other guns...). Of course you should use the laser rifle, the flamethrower, rocket launcher and the minigun as much and as long as you can. These are rare and very powerful. You can wipeout almost everything with those.
Apart from that use the guns I told you and don't try being cool with the EMW-56 Gauss gun. It is fun shooting around corners, but in order to lock onto a target you actually have to see them. This makes things more complicated on the Hard difficulty because you want to stay in cover most of the time.
The Kusanagi Assault rifle is in my opinion the best default weapon. This because of the secondary fire mode, which fires strong single bullets through light cover. This way you can stay BEHIND cover, use DART (RB) to see your enemies and kill them through the cover.

Cover
Don't rush! This is the most important thing to know in Syndicate. Enemies can spawn out of nowhere and if you rush - you will die. Best thing is to kill enemy by enemy slowly approaching and from distance. There are no unlimited spawns and not unlimited foes, so take your time.
Use Cover wisely and think about crouching (B)! I found the best way to stay in a secure place is to stay in cover, use DART (RB) to go for a quick attack and retreat to cover as to let your DART reload.

Chip upgrades
Choose them wisely! You can get only 8 or 9 I think. Choose those first which net you more health! This is utterly important for battles. Also, try to get the one where your DART system can be activated longer and maybe the one, where you can recover health by meleeing someone. This makes groups up to 4-5 people easy to defeat (just run around and melee them).

General fighting advice
Make good use of your abilities! Hack bots and strong enemies as soon as you can! If you have a miniboss who is surrounded by two foes, one blocking your hacking abilities, kill him first. If a strong enemy is surrounded by many guys, "persuade" some foe and hack the boss. Now you and your persuaded guy can attack him both. Persuade is the most valuable hack. Suicide becomes ineffective later on in the game (as it normally kills only one guy - the others will surround you), but the backfire ability with 3 targets can get very useful for recovering and regaining some time.
All in all, try not to feel too much like Rambo. For many boss fights, I just circled around some cover (the boss pursuing me) in order to regain health and recovering my DART system. This is by far the easiest method for most bosses, because you are alone with them in open areas with pillar-like indestructible cover.


I hope this helps and wish you guys out there good luck!

1) At the Menu select "Campaign"
2) "Chapter Select"

Here you will see a list of missions (if you've completed the game)
they will have "star ratings" next to them
1 Star = Easy
2 Star = Normal
3 Star = Hard

The missions with NO stars next to them are the missions you DONT have to complete on hard.

For those who can't see the list and who havn't completed the game the missions you NEED to do are:
1,2,3,4,5 - 7,8, - ,10,11 - 13,14,15 - 17,18,19 -

You DO NOT need to do missions:
6,9,12,16,20

**UPDATE**
You can do the missions in any order and you will still get the achievement.

Here is a link to a full guide to Hard mode achieving CEO rank on each mission with full commentary. It should help anyone struggling with the requirements.

http://www.youtube.com/playlist?list=PLC0818AE1A5C10126&...

Hope this helps