How about instead of ad hominem attacks you address the actual argument?

Assuming you go to mage classes at level 20 (any earlier won't really help) you'll be at mag 17 at level 20 with the minimum stats for mage classes.  By level 39 (I just chose that because that's where I am right now) your magic should be at about 23.  With Thoron (his best spell outside of aura which is too limited in uses) your atk is at 32.  Adding 3 for fiendish blow (3 instead of 6 to average out between player and enemy phases) you're at 35.  With tomefaire, 40.  Also, since this is a mag class, you don't have access to brave/killer weapons, no offensive combat skills, and your crest is now worthless.  I guess you could theoretically use magic weapons, but that seems to defeat the whole purpose.

On the map I'm at right now (ch 20) this would mean that you are offensively weaker than 33 out of the 42 enemies on the stage.  If we ignore the healers, then 31 out of 40.  In pretty much all cases, you're not just weaker, but weaker by at least 10 dmg.  And most of those enemies you do have a higher attack than compensate by using death, which at least has a good crit rate.  

The game is designed around the idea that your offensive units (and I doubt you'd be using a mage Dimitri as anything else since he gets terrible support spells and is not bulky enough in that class to be used as a tank) are going to be stronger than the enemies.  That's why they outnumber you by about 4 to 1.  I'd say we can objectively consider classify any unit who is so much weaker than the enemies (at recommended level) as bad.   Especially considering how useless they are compared to what they would be in another class, or if replaced by another unit.  

These, btw, are those enemies that you've complained are so weak that it makes the game completely devoid of challenge.  Considering that, I'm not sure how you can argue that Dimitri's magic would be "decent" by even the loosest definition of the word.  Please define what you mean by decent.  

As for, "not the best", it's not just not the best, it is actually the worst option by far.  If you don't think so, then please let me know what worse class you could choose for Dimitri.  If we want to be stupidly literal, then worst does fall under not the best, but if you say not the best when you mean the worst, then you suck at using language to convey your thoughts intelligibly, because that is not how anyone uses that phrase.

Good man.

I'm on chapter 17 of Blue Lions route...and holy cats is this emotionally taxing. My general rule with FE is to never allow characters to die...well since I only recruited Hilda and Dorothea that kinda became impossible. One one hand its definitely depressing having to kill characters I build supports with and gave random lost items to, despite not actually recruiting them...but on the other it feels like the most realistic war game I've played in the series as a result. Your enemies aren't just mindless NPCs, they're actually trusted friends and people you bonded with. 

That being said...any future playthrough I'm recruiting everyone possible. Already makes me sad to have to fight Dimitri and Dedue on any non-Lion playthroughs.

Potential spoilers, but assuming you do new game + it will be easier to recruit everyone.  You can use renown to upgrade your supports to what they were in the last playthrough, making it easier to recruit them, or use renown to increase Byleth's skills.  Of course, there are some characters that cannot be recruited on certain routes.  Also, keep in mind, if it's not to late, that certain people (Lysithia for instance) can be spared if you finish them off with Byleth.

About 15 hours in (other games have been distracting me). Overall, it's not the worst FE game I've played (Shadow Dragon/Fates) but it's not the best either (Awakening/Sacred Stones).

My main gripes are the embarrassingly bad graphics (they got Pokemon lazy here), the pacing and school faffing about makes it feel like I'm not nearly as far into the main storyline as I should be for the 15 hour mark. Also hate that the triangle system apparently is no longer a part of the series now(?) It's like the series is straying further from what made FE stand out against it's rpg competitors.

If the next entries end up like Three Houses I'm probably done with the series outside of playing as FE characters in Smash.

That's good to know about NG+ ...will make things much easier on my weak heart.

I restarted the stage because it got a bit too chaotic (even though I guess that's the point), but I think I can see a way to win with minimal named character casualties. If I have to kill that character I'll do with the MC though, thanks for the tip!

How about you read what I said? I'm not interesting in making walls of text over something dumb n pointless, especially when you're taking it way too seriously and aggressively. You're just gonna have to accept that.

I will get facts straight however, namely your incorrect claim that magic classes can't use brave/killer weapons. Any class can use any physical weapon provided they have the weapon rank for it (mounted classes can dismount to use fist). It's only magic that's class specific. Once learned skills can also be used on any class.

 I'm not behind your keyboard with a gun, you don't have to say anything at all.  But if you're not going to respond to the substantive points, don't make any comments about me personally.  Cause that's an ad hominem attack, and this is the second time you've done it.  And I'm really not sure how anything I said was aggressive.

As far as the second point, yes, technically you can access the weapons, but the hypothetical is about making Dimitri a magic class, and attacking with magic.  I would assume in this hypothetical, you're not also raising all his weapon skills, as that would kind of seem to defeat the purpose.  I mentioned magic weapons, so obviously I'm aware that you do actually have access to weapons, but I meant that in this scenario you wouldn't be using them, although fair enough, I misspoke slightly.  You could have actually taken the same time it took to point that out to respond to the general point on how this build could be considered decent, but prioritize how you like I guess.

Uh, no, lol. If you can't handle your person being put into question you're best off not on the internet. Also the purpose wasn't to attack but to explain why I wont engage in the argument with you, but take it however pleases you.

Again, time spent addressing me directly that could have been used just as easily to actually defend the point you were making.  Glad you find me so much more interesting to talk about.

I do find it very interesting how badly you wanna write walls o text over something so trivial haha.