Shambler is a new type of infected who first appears in The Last of Us 2. This page of the guide explains Shambler's unique attacks and how to eliminate Shamblers without receiving any damage.
The first clash with a Shambler will take place during the Seattle Day 1 section, specifically during the Tunnels stage after escaping from the television station building. As you squeeze through the ventilation shafts, you will witness a Shambler attacking a group of WLF members. When you get out of the shaft, you will be able to face these monsters for the first time.
The basic advice is that you should get rid of the "regular" infected first. If you do that, they won't interfere with your fight against a Shambler. If possible, try to stealth kill regular monsters, e.g. by approaching them from behind or by using a pistol with a silencer.
A Shambler's unique attack is an "explosion" that releases a cloud of poisonous gas. The most important thing is not to allow them to get close to you during fights. If you see a charging Shambler, start sprinting as soon as possible. Even if the monster decides to release the gas, you should be far enough to avoid any damage.
A rifle and a shotgun are great for attacking Shamblers. You don't have to target specific points on the monster's body.
Molotov cocktails and bombs are also a good idea - you can stop Shamblers with stun bombs and inflict damage with trap mines. Always be ready to use firearms immediately after a bomb or mine explodes.