While the forces of Cthulhu have access to advanced technology, not all of the entities use them. The reasons range from a lack of trust to a basic lack of intelligence. Just like the human armies with access to technology such as nuclear weapons, they are not entrusted to just anyone.
The most basic types of weapons utilized range from the (un)natural such as hand, claw, and tentacle to wielded weapons such as knives, swords, spears, etc. The fact that they are considered mundane does nothing to reduce how deadly they can be. There are creatures with enough strength to slice open a tank with its claws, or beings with tentacles that can crush an armored soldier with ease and yet others with maws so wide they can swallow a soldier whole. Likewise the wielded weapons are often made of a materials that are stronger than titanium and an edge measured in molecules that can cut through almost any material.
Many creatures also possess other natural defenses such as acid which can eat through the strongest armor, neurotoxin that can kill a human in a matter of moments, deadly venom, paralytic webbing, EMP bursts or any number of other abilities. Some of the most deadly entities faced by the forces of mankind do not use any form of technology or other augmented capabilities. To underestimate any creature within the army of Cthulhu is a fatal error.