The most obvious, prevalent issues would be moisture-retention and structural support. Not sure how to solve these though while still keeping them being recognizable jellyfish since we so few (dare I say virtually zero) working examples. The Portugese man-of-war is the only one I know of that has parts that are persistently dry and self-supported in the absence of water.
Other more insidious issues are related to inherent limitations in their morpholgy as we know it. The behaviours you describe would require very drastic changes due to these limitations. So much that you almost have free-reign so long as it doesn't seem completely unbelievable. The most immediate example that comes to my mind is the efficiency (or lack thereof) of their respiration and the apparent speed and strength limitations of their actuators, both of which are closely tied together.
Since you do not see any running, walking, or leaping molluscs, it would seem that actions such as leaping are reserved for animals that have some sort of rigid support such as vertebrates and arthropods.
For these reasons, I think the most implausible part of your description is how they somehow have the strength to leap such that their mass is sufficient to kill prey through blunt trauma. This is pretty implausible even for animals that exist today. It's a Catch-22. You need the square-cubed law so that your mass is enough that the impact is effective, but the square-cubed simultaneously limits the strength which you need launch yourself high enough. Not to mention that the predator is exposed to the same amount of deadly blunt trauma as the prey. I know of no animal that is capable of doing this, let alone one that actually does it unless predator dwarfs prey, in which case there are safer methods.
Compare this to ramming where you can build up momentum over time which does not require huge muscles capable of a very short, powerful impulse to launch your own body weight into the air at sufficient height to cause the damage you seek. Strength like that would allow for a very deadly toss of a much smaller object at much longer range.
You might want to take a look at are molluscs which, like Cnidarians, are moist soft-bodied creatures which are not tetrapods, except that some members are terrestrially adapted. They might be the only real-life example you have to work off of.