I find this weird that Jellyfish can avoid predators , hunt fish , keep in groups of jellyfish ( I don't know what there called ) and other various tasks which would seem impossible without a brain . Maybe its just the lack of research but how could they do these things ?

Expecially hunt fish.
Like for instance the some kinds of stings are like miniature harpoons with barbs on the end that inject poison to paralyze their prey. Some Jellyfish have sticky harpoons and others wrap their harpoons around their prey to trap it.

Any ideas to how they do this ?

I would bet not enough research. Maybe they do have a brain? Maybe it's just a jelly brain that we don't consider a brain? Just throwing out some ideas.

Trying to remember my highschool biology. A jellyfish is infact a group of cells without a central nervous system (no brain or spinal cord). Each of these harpoons you speak of are actaully single cells called nematocysts (or similar term) that are simply triggered to fire its sting by physical contact with its prey, nearly like a simple reflex action which requires no nerve impulses. Jellyfish are almost like a colony of different organisms (being cells) that all work together to achieve survival, where advanced organsims are also a collection of cells but controlled by the single being it composes. I still find it a bit difficult to imagine how jellyfish move and mate considering this but they do, i know some of them just drift and some are asexual. If you really wanna know more just google jellyfish.