This guide teaches you some basics, some tips and other helpful information tidbits to get you well on your way to blasting some heads open. This is my thoughts on the stuff that you may find helpful.
Some Basics for You
First of all, distance is your friend. There will be times throught the game where you won't have this luxury. Take advantage of having a space between you and your enemies. This space will allow you to really line up accurate shots and therefore become more adjusted to your play style.
I also recommend finding weapons which you are comfortable to play with and finding the best one to suit you. Check some other guides to see the differences between the weapons, and find the bread to your butter. This could be continued to say that you should adjust your equipment to how you play. If you prefer to camp in a room, you should probably choose more tripwires for the pesky ones who sneak through. If not, try dynamite and blow some more stuff up.
Don't be afraid to waste ammo, the game provides more than enough to get you through each area.
Finally, always recognise the different ways a zombie could sneak up on you. Spawns can be very unpredictable, meaning you should keep an eye on each possible way they could get to you.
Dealing with Larger Groups of Zombies
A large problem people have with Sniper Elite Difficulty is the non-stop hordes of enemies that will run you into a corner. The trick is to identify a path which you can make around the hordes so you have a little time and distance to pick them off one by one. This can be difficult in certain parts throughout the game, but this should be your Number one way of dealing with large groups.
Another strategy is to heavily revolve around secondary's and explosives. You can make it through large groups of enemies by using grenades on large groups and shredding through stragglers with a shotgun or a sub-machine gun. Then, use the rifle when distance is an option. I wouldn't recommend this strategy as it can be risky for running out of ammo.
I would highly recommend doing this with a couple of buddies, doing this allows for you and your team to section off certain enemy groups, and makes this difficulty much easier for dealing with special enemies i.e. Elites or Summoners.
Tip: Certain events through-out the trilogy will kill all spawned enemies if a main target is killed. Always try to focus specials first, because killing them may make situations much easier to handle.
Special Weapons and their Essential Uses
What makes this difficulty much less of a hassle is the efficient uses of special weapons, such as The Preacher and The Panzerfaust. If you can find and use each of these special weapons, zombies will be much less of a struggle and events will be much easier. Watch some videos, who shows where all locations are as seen here: