This will come in the first block and would be difficult to miss. You will be instructed to assassinate a target and need to approach slowly then press the hidden blade button that appears on screen.

I believe this means block one and will require you to kill all enemies in all four levels. Pressing the "orb" in the upper right corner will pull up a map that shows the locations of enemies in the level. Use this to ensure you have killed everyone.

This is unlocked at end of stage 7-1. At the beginning of 7-2 is an open area large enough to complete this. Start moving then jump pressing jump again each time you land. After the third flourish of an acrobatic jump, the achievement will unlock.

You will find blue orbs as you progress (red heal you, blue are used for upgrades). The orbs are in the level, sometimes rewarded from killing enemies and opening treasure chests (use the 'hand' icon to brush away the dirt then open the chest. You can upgrade both your health bar and sword at 75 orbs for the initial upgrade and 150 orbs for the second. The health bar can be upgraded again for 250 orbs.

I was able to upgrade the sword both levels and unlock the achievement either at the end of block five or beginning of block six. You may unlock it sooner or later depending on how thoroughly you collect orbs. I did not have enough orbs to upgrade the sword twice and health bar all three times in my one playthrough so keep this in mind. Upgrade your sword first and get it out of the way.

As you progress, you will unlock additional combos. These can be viewed by selecting the "scroll" in the top right corner. After completion of a level, it will show numerous different attack moves, but many of these appear to occur randomly depending on the enemy and the kill. The final combo viewable in the scroll menu unlocks after block 10-1.

The attacks are either light or heavy (left/right sword button respectively). The final combo (a counterattack) in the list involves guarding first by pressing the "shield" button near the control pad then the jump button.

The combos that appear to be required are as follows (all but the first two are visible in the scroll menu):

Light, Light, Light
Heavy, Heavy
Light, Light, Heavy
Light, Heavy, Heavy
Heavy, Heavy, Light
Block, Jump

The bombs are unlocked at the end of memory block five. Bombs are selected by opening the weapon menu (middle left, usually an empty hand). If you are standing still and press the bomb button you will drop the bomb at your feet (or off the ledge to the ground below you). If you are running you will the throw the bomb in the direction you are running. You can carry five bombs at a time and they are refilled at the beginning of a new level or by finding a crate. Bombs are not available in all levels even after unlocking them. The blast radius is quite large making it easy to kill enemies with the bombs and a well-placed bomb can take out multiple enemies.

Throwing knives are selected in the same manner as the bombs. These unlock during memory block five. They are difficult to aim, traveling in the very specific direction you are facing. If you are angled slightly, you will miss your target. This can be annoying, but it is still not difficult to get the achievement considering the number of enemies. You can carry fifteen knives and they are refilled like the bombs. Likewise, you do not have access to the throwing knives in all levels.

The finger blade (or hidden blade) is available from the first memory block and is available at all times. Many enemies will be seen with "Z"s above their head indicating they are sleeping. Walk slowly up to them and the hidden blade icon will appear. There are numerous opportunities for finger blade kills.

The crossbow is unlocked at the end of level 8-2. You can carry fifteen bolts. The crossbow has an auto-aim feature denoted by a dotted line and targeting reticule. If the line is green you have a shot, if it is red you don't. You cannot move while the crossbow is equipped, however you can rotate Altair to locate or change your target. At the beginning of 8-3, you will be prompted to use your crossbow to kill a group of five guards. Do this and the achievement will unlock.

There is some confusion regarding this achievement. Memory block thirteen has only one stage with numerous enemies and highly accurate Templar crossbowmen making it almost impossible not to be hit. I was hit numerous times but on defeating the final boss he dropped a red orb which fully restored my health. The achievement unlocked at the completion of the level. It appears that you only need to be at full health at the end of the level to unlock the achievement. Try not to be too low on health prior to defeating the final boss and this *should* unlock.