How do you convince someone that has studied the Biblical languages who says EU is wrong and eternal punishment is the correct doctrine that EU is the correct doctrine?
If they really have studied the Biblical languages, then they should at least recognize that in Hebrew OLM and AHD do not always refer to never-ending periods of time as a demonstrable fact;
and that the Greek adjective eonian therefore also doesn’t in the LXX, plus at least once in the NT (at the end of Romans);
and that the typical expression of OLM, since Hebrew has no adjectives really, is l’OLM, (in)to the horizon-limit (a horizontal distance metaphor), rephrased in Greek as {eis aion}, in(to) the eon and its several cognate forms;
therefore neither do those various prepositional phrases in Greek necessarily refer in every case to never-ending continuance;
therefore also neither do the related Aramaic/Syriac terms of the NT in that language (or languages since there are a couple of forms across the centuries, relevant to NT textual studies, completely aside from whether they represent the more primitive NT language than Greek to any degree);
therefore also neither should Latin translations from the Greek and/or Aramaic, whether OT or NT (although this should be especially obvious in the Latin OT);
therefore also neither should any other less common ancient textual studies language for the NT like Coptic (primarily) or Ethiopic (which is similar to Aramaic) etc.;
consequently, that any argument depending on the presence of the adjective or prepositional phrase depends primarily on thematic and narrative context arguments, immediate, local, and extended;
therefore, they should already recognize that the contextual arguments, not the terminological (“Biblical Language”) arguments, are the main exegetical testimony, for or against any soteriology.
(Note that this is aside from disputes about whether the adjective and/or prepositional form never refers to never-ending continuance. But even if they always referred to that in principle, in practice the reference is sometimes used for rhetorical exaggeration or is taken back by God at a later date – e.g. maybe God intended at least theoretically for Aaron’s family to have the priesthood forever, which God Himself reminds people when He’s in the middle of canceling that privilege! – which is contextual evidence, not mere linguistic evidence.)
If they don’t already recognize that, then they should study Biblical languages somewhat more first. 
Even if they’ve spent decades on it, and can teach it at a doctorate level, they’ve missed that part somehow, which is demonstrable enough.
Consequently rarer words which might refer to never-ending continuance, like {aidios} (which can also refer to invisible things), depend on immediate, local, and extended contextual arguments, too, for exegetical weight.
To this I would add that soteriology depends logically on theology, and so any soteriology that logically contradicts the overarching theology indicates a serious mistake somewhere, whether that’s with the theology or with the soteriology or perhaps with the exegesis. This is how I got into universal salvation to begin with (along with some other members here): I was working on my trinitarian theology, and came to realize that the implications of trinitarian theology point toward at least a minimum universal salvation being true (where God constantly acts toward saving all sinners from sin, which an ongoing stalemate would not contradict.)
That would depend on what specific points they are making in support of their erroneous opinion.
You could direct them to the Greek scholars of the Early Church Fathers who were universalists.
Thanks. I plan on diving into the Biblical languages down the road. Any advice for me?
I love languages other than English, but I also suck at them. So… “pray”? 
Take modern Greek at a junior college - for 2 years (if they offer it). This applies to US citizens and residents. And find the equivalent, in another country. Then it’s easier to pick up ancient Greek. Hebrew then can be added after 2 years - as an additional language. Because by then, you will know how to learn languages - for the most part.
If it comes to that, Duolingo is a great free way to learn modern Greek. (And modern Hebrew.) It’s basically the Rosetta Stone but free. (They earn money from occasional ads, and from treating the scoring like a game so if you want to keep your scoring high compared to other players you can pay a little money.)
Yes, Jason. You are right. And if you have an ad blocker installed in the browser…They don’t have software yet - to detect it. I use Duolingo to learn multiple languages. But I also had 2 years, of College French and Spanish. Which gives me a considerable head start.
I did go though the Duolingo Russian training. My only complaint is that they didn’t cover the Russian alphabet and pronunciation. But you can pick this up quite easily - in Russian for Free and other sites.
Soon, Doulingo will have a Japanese course. But by then, I’ll be very familiar, with the 3 - 4 writing systems (if you include the Roman pronunciation system).
The biggest hurdle with Greek, is that they have a different alphabet. And I believe that Hebrew, works from right to left. But you will find both these offerings - in Duolingo. And for some strange reason, they are also developing Star Trek Kiingon. Perhaps this might be useful, in following the new Star Trek Discovery series? Where the Klingon empire, is now enemies with the Federation. 
And talk about BIG decisions. CBS ran over last night - with the Sixty Minutes program. Which meant that Star Trek Discovery and AMC Fear the Walking Dead - were on at the same time. Which one do I watch now and which do I watch later? Well, there were a hoard of Zombies attacking. So that won out - for my immediate watching. 
Here’s a review, of the new series:
Star trek: discovery - series premiere review
And in case you wish to learn Klingon - here we go 
Just in case you meet an alien: Here’s where to learn Klingon
Btw, James, I updated your topical title so that members who might want to discuss the question will know what the thread’s about. 