Steam workshop map you'll need: https://steamcommunity.com/sharedfiles/filedetails/?id=18651...
(All credits to the map creator: Jared)
Step 1: Subscribe to the map I have linked above
Step 2: Load up the map in csgo. It will be under workshop maps (make sure to select easy bots. selecting harmless bots will disable achievements)
Step 3: The map is very self-explanatory, just kill the bots with molotovs and grenades until the achievements pop
This will unlock both 'Flame Expert' and 'HE Grenade Expert' achievements