Devil May Cry 4

This achievement requires you to max out your health bar.
There is three things you need to do to max it out:

Buy them from the shop.
There is 6 blue orbs to buy from the shop, this should cost 113,000 Red Orbs.

Complete all Secret Missions.
The only really annoying mission would have to be mission 8, very very frustrating.
Doing this should also net you the "Nothing left unsaid" achievement too.
Guide: http://www.xbox360achievements.org/forum/showthread.php?t=56... (Credits for guide goes to Demon9Tom of x360a)

Find all Blue Orb Fragments.
This is rather easy, the only one worth noting is the one on Mission 12, as it is quite a bit out the way.
Guide http://www.xbox360achievements.org/forum/showthread.php?t=57... (Credits for guide goes to matty cheetham of x360a)

By now you should have a full Vitality bar.
If not, i would advise checking to see if you have collected all the blue orb fragments.

To get this achievement you must gather all the blue orb fragments, raising your health bar to its maximum. You will have to buy some, win some by completing secret missions, and find some hidden (or not-so-hidden) in the stages.

Since there are guides to the Secret Missions in solutions to Nothing Left Unsaid, I'm only going to provide material for finding the blue orb fragments in the stages, NOT for the secret missions:

http://sephirosuy.blogspot.com/2008/02/dmc4-blue-orbs.html

This is the most useful guide I've found for it, with pictures and a few videos. I had nothing to do with its creation, I'm just providing it for those who can't find the orb fragments from descriptions alone (something I had a lot of trouble with).

Again, check the Nothing Left Unsaid solutions for the secret mission locations and completion.