This is an interesting perspective. However I am concerned about the fact that none of my players have ever encountered a mechanic like this before. If I don't give them some hints they will simply be frustrated by the encounter. I'm not looking to make this a trivial problem for them to solve. Just to give them a fighting chance.

@linksassin that's not really the question you're asking, though. 'How do I inform my players of common dnd workarounds or expert player tactics without simply telling them' is not the same as 'I want to ensure my players defeat X spell'. In general, people figure out these tactics by either seeing other people use them or being confronted with a problem they need to actually solve - the latter of which you are removing by 'pre-solving' the problem for them. Perhaps your players want their problems pre-solved - but again, this is a different question to ask.

@user2754 I think your comment better answers OP's question than your actual answer. You're basically turning this challenge into "memorize the rulebook or the villain escapes". There are other ways to make the challenge solvable without pre-solving it for them.

@user2754 I asked how to prepare them for it, not how to ensure they defeat it. I'm not looking to "pre-solve" the problem. But to give them the information they require to have a chance at solving it. I believe in never giving my players a challenge they don't have a chance of solving.

@linksassin I genuinely have no idea what you're talking about. You don't use knowledge skill rolls in your game? Or that's not.. enough information? Your players don't like poke things with poles or toss rocks at it if they don't know what the big glowy wall does? I've made some assumptions in my answer that have in general been true for any group or game i've ever even seen or heard of as regards interaction/induction/following basic game rules but your comments are indicating they might not be true for your group?

@user2754 I believe they will throw rocks at it and attempt a variety of thing. However this particular spell requires very specific spells to be able to defeat. Without forewarning it is extremely unlikely they will try the correct combination or even have them available.

@linksassin "You succeed on your Spellcraft/Knowledge: Arcana roll, and identify it as the Prismatic Wall spell." opens the PHB to the relevant page, and shows it to the players "Here's how it works."

@linksassin if you don't allow knowledge checks to grant specific information at the listed DCs in your game, you need to notate that in your question, as that is a house rule that DEEPLY changes how the game works and any and all answers will likely be answering a completely different question than you think you are asking

Again, that's really not what I'm saying. Sure, once the spell is cast they can make a check. Should they succeed I give them the information. But damn... they didn't prep the spells they needed because they weren't given any forewarning. Therefore it is an impossible challenge and they have no chance of succeeding. That is exactly the situation this question is trying to avoid.

"Allow a Knowledge: Arcana check to ...." would be a valid answer to this question. Please note I have deliberately left my own ideas of how to handle this out of the question so as to promote answers. I am aware of things like allowing a skill check to provide the information.

@user2754 I'm sorry if my comments have confused the situation. I will restate the question to try to clarify.

I don't like to simply give my players the answer via a high knowledge roll but prefer to provide them with all the knowledge, skills and materials they need through gameplay.

This question is asking, "How can I prepare them for this encounter to give them a better chance of success, without simply telling them it is coming?"

This is about ensuring the encounter is enjoyable rather than frustrating for my players. I want to avoid a situation where my players say "How were we meant to defeat that?"