This guide for Serious Sam HD: The Second Encounter will provide locations of all 149 secrets in the game, including the secrets in the DLC. 

I found this achievement very tricky and there are big chances for you to miss extra life so here are some locations. There are 13 extra life items in Cooperative Coin-op game. Special thanks to Chrissyx for sharing some screenshots which helped a lot 

Want to get gold medals on all cooperative survival maps but don`t know how? 

A video compilation showing where you can find all secrets. 

This is a guide on how to beat the 6:00 record on Kleer base solo the easy way!