How to make heros stronger faster?

There are 2 ways to make heroes stronger. You can level them up, this takes Antiserum which you can get from the virology lab buildings and from attacking zombies and Zombie lairs in the world. The higher level level of zombie lairs you defeat the more you will earn, and the mor you can do each day the faster you can level up. At certain levels the hero will need to be 'Evolved', This also requires different levels of Combat manuals which yo get from completing campaign missions. You will need to do a lot of these missions (repeating them as necessary) to earn the manuals. especially combat manual III.

Heroes can also be Enhanced. This will add to their stats a little and also increase the effectiveness of their Troop skills. To do this you need to get frags (hero fragments) for the specific hero. These are the same as when you recruit the heroes. Getting these faster will depend on the hero. Some are earned in the tavern, some you will need to compete events to get.