I was stuck on the funeral mission for a little bit, because I am quite the perfectionist (I wanted the achievement where you kill all targets with a full DNA bar)
Suffice to say, doing so against a flock of Templars, guards, and feaux-Robert is not easy. Luckily, I managed to beat the level in about 3 seconds.
I had my hidden blade selected, and as soon as I exited the funeral cutscene, I made a beeline to "Robert". Where I was, she had to climb the fountain and jump off to come at me in a straight line (which she did, quite unexpectedly), and as soon as she landed, I pounced.
Never had to even touch one of the guard templars (though I made a point of killing them), and was home free. It was very satisfying after the irritation of trying to fight them all off in the streets, and before that, dealing with the docks.
After that, a straightshot to the end of the game! --- I am the rocks of the eternal shore...crash against me and be broken! GT: Deadly Mantodea
Killing all targets with a Full DNA bar refers to side objectives (Viewpoints, Save the Citizens, Investigations), not to health (which is your Synchronization bar).
edit: to clarify, this is what I mean.
When you are going through the game, you are technically supposed to do all the side objectives as mentioned above (the flags don't count, they're on their own bar in the DNA menu) before going for the Kill.
That being said, if you missed any and haven't finished Sequence 6 yet, you can go back to each Memory Block, do the ones you missed, and it will still count for the Achievement.
That being said, the Achievement is glitched. Some people don't do all the requirements for it, and get it. Some people do all the requirements for it, and DON'T get it. --- Behold the angry wizard putt-putt-putting away.
Ohhhh, I gotcha! Luckily, I was anal about that as well, and always did all of the missions before going for the targets. Might as well, as they do give good info.
Speaking of glitches, I got the achievement for doing all of the save-citizen missions for the last Jerusalem district when there was still one left. --- I am the rocks of the eternal shore...crash against me and be broken! GT: Deadly Mantodea
Fighting Maria (that's her name) at the funeral can be a pain with all those guards. Some people might get lucky and quickly get a counter with the hidden blade or run behind her. A more reliable quick death is... http://www.youtube.com/watch?v=w-vYBX8cngs&feature=player_detailpage --- Visit thehiddenblade.com for expert gameplay videos in Assassins Creed; which includes speed, stealth, style and finding things that shouldn't be possible.