This page of our trophy guide to Sniper Elite 4 contains a detailed description of Gotta Cap 'em All trophy. You can get this trophy for killing every type of infantry opponent.
There are eight types of enemies in the game. They were all described in the game's tutorial, complete with illustrations. In order to unlock this achievement, you just have to kill a soldier of each type, at least once. It's hard not to get this one, unless your approach in the game is extremely stealthy. Of course, enemies killed throughout the whole game count - there is no level that would contain all types.
List of types of enemies:
- Soldier with rocket launcher,
- Supprt Jager (armored, with heavy machine guns),
- Observer (characteristic binocular icon after targeting),
- Radio operator (carry radios in their backpacks).