If you're planning on through playing easy difficulty first and using a second playthrough for normal and Apocalyptic difficulty, you'll probably find that the two runs are of similar difficulty. This is because you can collect the Abyssal Armour and use it in your second playthough to make things much easier. If you do play Apocalyptic first, this achievement and the normal achievement will also unlock when you complete the game.
Here's some spoiler free advice to help you through the game:
Collect life and wrath shards as early as possible!
I know a lot of people like to save collectibles until they've completed a game, but it's best at least to use a guide to ensure you've always got the highest possible health. This is especially important early in the game.
Stoneskin is perhaps the most useful of the four wrath abilities. Levelled up, it gives very good damage protection and a boost to the damage you deal. It's not flashy, but it's great for boss fights and prolonged battles as it lasts for a long time with low wrath consumption. If pure offence is more your thing, check out Affliction, but its value is often limited against bosses and it doesn't appear until later in the game.
Know your weapons!
Most people will want to stick to the Scythe and Tremor Gauntlet for levelling purposes. That's fine, but realise your scythe is best used when facing a large amount of weak enemies and your gauntlet is great against tougher enemies and when close to cliffs (knock the enemies over the edge). The RB+Y upgrade is essential for both weapons, so buy those early. It's quite alright to only use Chaoseater when you need to build up your chaos meter - there's little benefit to levelling it up, as you'll get a new sword later in the game.
Equip the right bonuses!
Slotted enhancements that improve defence, damage and weapon experience are the most useful. Remember that some enhancements will provide a passive bonus by simply having them in your inventory. War's Glory and Weapon Master are good examples. Know that some enhancements are better paired with certain weapons. Further reading, but risk of spoilers, here: http://darksiders.wikia.com/wiki/Legendary_Enhancement
and here: http://darksiders.wikia.com/wiki/Category:Enhancements
Learn to evade!
Blocking is useful, but many times you'll find that enemies are still able to damage you. Getting a few hits in and then dashing away (direction + RB) is a much more effective way of avoiding damage. Most enemies have a pattern to their attacks, so if you're taking a lot of damage, change your timing. This is especially true for the bosses.
Don't use green and yellow chests until you need them!
Regular green and yellow glowing chests contain health and wrath. If you're already almost full, the contents of those chests will be wasted. You will be required to revisit or backtrack to many areas of the game and these chests are usually positioned near the site of a big fight - if you're good for health and see these chests, be prepared for a fight when you return to his area. Of course, don't confuse these with green and yellow chests with black magic swirling around them - these are life and wrath shards and should be collected as soon as possible, along with blue chests and bone chests.
Save your Chaos Form for when you *really* need it!
Often I find I'll go into a big fight and trigger Chaos Form, only to have it run out too early or have it cancelled by a cut scene. Attempt all battles with your regular weapons first and use the chaos as a last resort. Chaos Form is useless against most bosses, so don't even bother.