The first encounter with the Breaker boss can be completed fairly easily by using the passage under the rocks immediately right of the mission start location back and forth while shooting or tossing Molotov cocktails at the Breaker as it tries to follow by walking around the rocks.
One trick to clear out all infestations without bothering about fuel or bike damage is to take a detour mid-mission when the game instructs you to go to a certain place and instead go off and destroy all the possible freaker nests. The game is set to mission mode meaning that no random events will show up and distract the player as events usually do when roaming freely. Perhaps the only drawback is that depending on the mission the player might run into a "leaving the area" message such that the range is sometimes limited.
The safest way to defeat the Chemult freak horde is to climb up on the roof of the small shed after the supermarket, attract the horde by shooting the car alarm and then tossing molotov and napalm cocktails off the back side of the roof.
One pitfall is that for a very efficient playthrough, the player should follow the horde closely through the supermarket because if the horde manages to get out of range it is sometimes difficult to attract the horde to come back to the wooden shed. The player should wait for all the freaks to gather inside the shed and only then start throwing down the cocktails to burn as many freaks as possible in one single go.
Nevertheless, the wooden shed seems to be the safest spot known so far because it offers a good overview of the freaks gathering underneath and the freaks are also unable to climb up and reach the player no matter how many corpses pile up at the entrance of the shed.