This achievment can be completed easily, it may take a couple tries however as 101 cremations with 1 molotov is unlikely.
It is easier to get more kills with molotovs when there are a large number of hordes around. In particular:
~After calling a rescue
~After being vomited on
~ Setting a car alarm off
~ or at a stage where a sound is set off or motion which attracts the horde.
To kill them you can use either a molotov or petrol can; (found lying around - throw and shoot to explode) or shooting gas tanks. These all emit fire which are deadly to the infected and are most effective when the horde are alerted.