Dead Space 2

(Some tips in this solution will be restated from my solution for "Epic Tier 3 Engineer" in Dead Space. Some tips are specific to Dead Space 2.)

As you know, Zealot difficulty is the hardest difficulty (in a sense) in Dead Space 2. You will take massive damage from enemies, and you'll find almost half as much ammo and loot than you would on any other difficulty. This means that you have to play carefully and make every shot count!

There's two ways you can go about playing Zealot:

1) You can import your file from your first playthrough and begin a Zealot playthrough with it. Doing this will start you off with everything you had in your previous playthrough so you will most likely have a fully upgraded RIG, and at least one fully upgraded weapon. Zealot will still be difficult, but your odds for survival are significantly improved.

2) You can begin a playthrough of Zealot difficulty from scratch. I won't judge, but in my opinion this is the most satisfying way to go about it.

(This guide assumes the player took the second option, and owns no DLC)

(If you don't own Dead Space: Ignition I highly recommend you purchase it. Having a save file will unlock many Conduit Rooms throughout the game that are full of goodies such as power nodes, medkits, and exclusive weapon skins and the hacker suit, that can make the hacking mini-game quicker.)
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Finally, the tips:

[Your Loadout]

Weapons are down to personal opinion, but there is no doubting the usefulness of two weapons: the Plasma Cutter and the Contact Beam.

- The Plasma Cutter is your bread and butter gun, and should be what you use most of the time. It's just as powerful as it was in the previous game, with an added ability to ignite enemies when you upgrade it's special.

- The Contact Beam is an absolutely devastating weapon that will kill ANY necromorph besides bosses and/or the Brute in one shot. This INCLUDES enhanced necromorphs. This gun is a must-have for Zealot difficulty because you can use it to take down tough enemies and save many plasma cutter rounds in the progress. It also has a very
handy alt-fire that will knock down any enemy that is too close to you. What's better is that while doing the animation, you are invincible. Be wary, however that with this weapon you are essentially firing money because you will only find ammo in stacks of 1, and they are worth 1,000 credits. Make your shots count, and only use them when you really need to. With this weapon, the final boss is a joke.

These are the ONLY two weapons you need for your Zealot run. The other weapons have their uses, but they are either situational or just plain useless on Zealot due to the high enemy damage resistance.

[Suits]

A new feature in Dead Space 2 is the special properties that suits have. No matter what suit you are currently using you will always retain your highest unlocked armor and inventory space. So you can use the Engineer suit yet still have the full benefits (minus it's special) of the advanced suit.
Which suit you use is down to personal opinion and preference. If you own Dead Space: Ignition, using the Hacker Suit is useful to make the hacking mini-game faster.

I personally recommend using the Vintage Suit until unlocking the Advanced Suit.
Although the Vintage Suit's credit discount is small, it adds up. Every little bit helps!
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[Upgrades]
(Keep at least one power node on you AT ALL TIMES! Power Node rooms are always worth opening.)

- Like in the previous game, max your Plasma Cutter's damage ASAP. Also, the quicker you unlock it's special, the better. Once your Cutter's damage is maxed and it's special unlocked, then you can mop up it's remaining upgrades.

- Another good thing about the Contact Beam is that it doesn't have any damage upgrades because it already kills in one shot. However, it has a very important special that can help greatly, so try to obtain it prior to the final boss. Upgrading it's reload speed and charge time can make it easier to use and thus are good upgrades to get.

[RIG, ETC]

[HP] - Very important upgrade. You should upgrade your HP regularly. Also note that when you upgrade your HP it will fully restore it. Good to keep in mind. Also if you are rich, you can refund a HP upgrade (once refund is unlocked) and purchase it again. Not recommended unless you have nothing better to do with your credits.

[Air] - Ignore. Virtually all vacuum sections have floating oxygen tanks or oxygen stations littered around. Upgrading it once is all you need if that.

[TK Damage] - This one is a mixed bag. On it's own, TK is very useful in this game, and it already packs a punch by itself. Upgrading it once or twice is fine, especially if you have your important upgrades out of the way.
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[Energy, Duration, Recharge] - All useful, no reason not to max these. Upgrading energy will fully recharge it. Even though in this game your energy will passively regenerate on it's own it's still worth a note.

[Kinesis] - Useful and cheap to upgrade. Upgrading Kinesis will allow you to grab limbs and other throwables from further away. It will also help bring out of reach items to you. Upgrade whenever you feel the need.
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[Combat Tips]

[Go for the Limbs!] - Obviously, the fastest way to dispatch an enemy is to cut it to pieces. Always go for the limbs! Also, did you know that if a Slasher looses both of it's arms it will instantly die? Good thing to keep in mind.

[DON'T underestimate melee combat] - Unlike the first game, melee combat is a viable, although risky combat option. Best used against one on one encounters. Stasis an enemy and wail on it with your fists, or stomp it while it's down. These are great ways to conserve ammo. Especially effective against crawling Slashers.

[Use TK whenever possible] - In Dead Space 2, throwing objects at enemies is very effective. Keep an eye out for red explosive canisters to hurl at unsuspecting enemies.
Just as important, use the limbs of dead enemies against living(ish) enemies! Although it could take both arms of a slasher to kill another, it will still harm them and save you ammo, especially in the early stages of the game. Use TK against enemies whenever possible.

[Conserve your Contact Beam for big game] - As I said earlier, the Contact beam will kill all non-boss enemies in a single shot. Use this to your advantage, saving your ammo for enhanced enemies or brutes. Also keep it's alt-fire in mind, it's very useful when you get surrounded.

[Don't forget about Stasis] - Stasis might not feel like it packs as big of a punch as it did in the first game, however Stasis is still vital. You can use it a little more freely in Dead Space 2 since it will now slowly recharge on it's own, but still save it for situations where it's needed. You can also use it to stasis lone enemies so you can rough them up with melee combat to save ammo.
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[Other Tips]

[Use the safe] - The safe is your friend, please use it. Instead of selling excess medkits and ammo, store them in the safe for later. This is essentially giving you some insurance for the future in case things go badly. You won't have to spend precious credits to pick yourself back up when you have items safely in storage.

[Don't be lazy] - I know it's tedious sometimes, but backtracking helps you. When your inventory fills up, backtrack to the store to sell/store away items. In Zealot, you get a fraction of the ammo that you get on other difficulties, so every little bit helps. Everything is worth credits and you'll need all you can get.

[When to use Large Medkits] - As you know, using a large med kit fully restores your HP. This is wasted if your HP isn't upgraded a couple times, since you can use a combination of mediums and smalls to fully heal you. It's best to use a large medkit when your health is orange/red. This can prove difficult on Zealot since you will die quickly. Use your judgement. The best situation is after the threat is gone, and your HP is low. Giving you enough time to access your inventory and use it. It's far more effective to use a large to fully restore your health when it's low, then to use 3+ meds and a small or two. (If your HP is fully upgraded) Save these for when you need to patch yourself up.

[Having trouble with the final boss?] - Not with the Contact Beam you aren't. This gun will take care of the boss in a few shots. You can use it's alt-fire when the phantoms get too close to blow them away.