So my preliminary answer to the question in the thread title would be:
How would you respond to an existentialist who says
1 - "You Epicureans have chosen pleasure as your meaning but it's not universal"
Yes, I can agree that not everyone chooses to hold pleasure to be the meaning of life.
2 - Do Epicureans hold that pleasure is the universal Good?
That requires being clear about what "universal good" is supposed to mean. Epicurus held that Nature has given us only pleasure and pain for determination of what to choose and to avoid. If you accept "universal good" to mean "Nature's directive to all living things" then yes "pleasure" fits that bill. If you want to suggest that there is some other definition of "good" other than the directive Nature gives to all living things, then you've got an entirely different ballgame of persuading that your definitions of "good" and "universal good" are correct.
Edit: In my answer I am attempting to parrot Torquatus, as I am becoming more and more persuaded that the parts Cicero gave to Torquatus to say are probably among the most highly developed statements of Epicurean philosophy that exist. I would see whatever Cicero's source was as along the same lines as Lucretius, representing 200 years more work since the time of Epicurus to refine these formulations:
[30] Every creature, as soon as it is born, seeks after pleasure and delights therein as in its supreme good, while it recoils from pain as its supreme evil, and banishes that, so far as it can, from its own presence, and this it does while still uncorrupted, and while nature herself prompts unbiased and unaffected decisions. So he says we need no reasoning or debate to shew why pleasure is matter for desire, pain for aversion. These facts he thinks are simply perceived, just as the fact that fire is hot, snow is white, and honey sweet, no one of which facts are we bound to support by elaborate arguments; it is enough merely to draw attention to the fact; and there is a difference between proof and formal argument on the one hand and a slight hint and direction of the attention on the other; the one process reveals to us mysteries and things under a veil, so to speak; the other enables us to pronounce upon patent and evident facts. Moreover, seeing that if you deprive a man of his senses there is nothing left to him, it is inevitable that nature herself should be the arbiter of what is in accord with or opposed to nature. Now what facts does she grasp or with what facts is her decision to seek or avoid any particular thing concerned, unless the facts of pleasure and pain?