The instant kill with (X) heals you, and it works on different types of enemies, including armored Giants.
Sniper headshots is a viable way to kill flamers and gunners.
The difficulty in single player is very uneven. It is trivially easy, except for the blood fountains, which are unreasonably difficult. They take place in tiny arenas with swarms of enemies, of which you have to kill a set number. Health is saved for every checkpoint, including blood fountains, so you can start these challenges with almost no health. If it is too difficult, your only option is to start the entire level over.
The levels of this game are very open, and seems designed to work well for cooperative multiplayer. Although the game seems to be based on the Sniper Elite series, and your main weapon is a sniper rifle, the sniping itself rarely takes place on long distance. It is more similar to the Gears of War series, where sniper rifles are high damage weapons used to ensure getting headshots on enemies. The enemy classes remind me a lot of Gears of War as well, you have gunners, flamers, shielded grenade throwers, etc.
This seems to be a silly zombie shooter based on the Sniper Elite series. I feel like the sniping was the best part of the Sniper Elite series, but this game is not really focused on sniping, but more on shooting wildly into crowds of zombies.
I was playing through the zoo level late in the night, and reached the blood fountain. I had very low health, which I learned was stored in the checkpoint. The level was pretty long, so even though the blood fountain seemed way to difficult, I pressed on. I ended up playing this checkpoint over and over again for two hours straight, learning all sorts of mechanics. I ended up using Divine grenades to heal myself, run around the tiny arena a lot, climb onto a platform and throw fire grenades down, and carefully taking flamers and gunners before they could kill me. At 3 in the morning, I finally beat the level.