Devils get Magic Resistance. Is there a good tactic to counter that as a Sorcerer or Wizard?

Yes.

Summon Lesser Demon

summon greater demon

Magic Resistance just gives advantage on saving throws.  Use attack spells.

There appears to be a dearth if attack spells that are not fire or poison?

*shrug* I suppose, but there's some.  Ray of Frost, Magic Stone, Ice Knife (attack and save, but still), Chill Touch, Chromatic Orb, Magic Missile, Witch Bolt, blah blah blah.  I guess a lot of the non-fire/poison ones are cold, which they're resistant to, but... I dunno.  There's enough, most wizards should have some.  Chromatic Orb at least.

Chaos Bolt is always a good spell, even if the dam type is random, it has ones in there that can be effective too.

sometimes a wizard/sorcerers best bet, is to just cast haste on their allies. Or slow on their enemies when they aren’t magic resistant.

though I get the sense from the thread you’re more of a Blaster caster. And so might not have many utility, support, buff, debuff spells?

Is this for a sorcerer or wizard specifically, because they both have different ways to go about it that don’t overlap necessarily, as such with summoning a demon to fight the devil for you.

So as I'm now at a computer rather than phone some more details which might help.

I'm playing BG: Avernus and we are just about to head into Hell.  We have a chance due to story/campaign reasons to reconsider our characters and maybe roll afresh.   We are 5th level.   I really fancy some sort of spell caster - and usually for me that means a blaster of sorts.  But I am aware Devils (I assume these will be primary but not only foes) are immune to fire and poison, plus resistant to cold.  They are also Magic Resistant.  So blasting spells are pretty limited, unless I can come up with some other approach or trick.

I can obviously use Buff spells, but Debuff and Control are naturally much more limited due to MR.  So I am trying to work out a theme/angle.   One idea was to specialise in Cold spells (I like a theme where possible), and take Elemental Adept feat.  But I'm still really limited an actual useful spells there.  Maybe I simply use Frost Bolt on any devils we fight and save the other frost spells for non-fiends?

In general I am just after concepts, ideas and thoughts on how this might work, and not be utterly ineffective...

TIA for any further responses!
Blakey

I made a warlock that was specialized as a demon hunter.

Used mostly the summon demon spells listed above. As staying back in my protective circle of blood. One of the few warlocks who doesn’t have anything to improve eldritch blast. As the mom maxing for that didn’t fit the thematic approach of a demon hunter.

Edit: not all debuffs, need to target your enemy. You can debuff terrain and other conditions to create more optimal fighting environments. Maybe using shatter to cause a bunch of stalactites to fall and impale. 

If you’re considering being an elemental specialist have you considered thunder/lightning specialties?

Divine Soul Sorcerer gets to choose Spiritual Weapon, and if you're evil, you also get Inflict Wounds.  

Both Sorcerer and Wizard get the Animate Objects, that's a great way to bypass an enemy's advantage on saving throws.

I don't think that the creatures created by Animate Objects deal magic damage, so Devils would be immune to them.

 Get a blacksmith in town to make you a bunch of silvered Steel balls. Take them with you and cast Animate Objects on them. Obviously, DM would need to be on board with it, and it'll probably cost you, but very effective against devils. Players Handbook calls out a price of 100gp for 10 pieces of silvered ammunition (which could be considered the tiny objects in the spell description)

Quote from Blakey >>

Devils get Magic Resistance. Is there a good tactic to counter that as a Sorcerer or Wizard?

 Sure. Offer to make a deal. Devils love bargains. 

More seriously, there's lots of tactics. First is to just acknowledge that the devils will make their save, and cast the spells anyways. Don't use Save or Die spells, use ones like Fireball that still have an effect on a successful save. Well, not Fireball, because fire damage immunity, but Lightning Bolt or the like.

Another idea is to just use spells that don't call for saves - Bigby's Hand is one of my favorites as a wizard. 

Don't attack at all, and buff your teammates. Cast Fly, or Haste, or the like. Sorcerers love Twinning Haste on their teammates. Or toss up Walls or something. You don't have to attack all the time. 

Save your spells for Counterspell and Dispel Magic. Devils tend to come with nasty spells of their own, or accompanied by warlocks, etc.