Killing insect creatures such as vespoids and hornetaur is the only way to obtain monster fluid. Try slayer quests related to insects and use a poison type weapon so you may get a chance to carve the body. Avoid poison type weapons with high damage such as longswords and instead use qicker weapons such as sword and shield. The feyline skill Dismantle Hi is also useful as there is a chance you will get two carves on a single insect body. For best results try fighting in the jungle zone as there are the most insects.