Magic Items as a Temporary Fix
As per other answers, giving magical items as a fix may just paper over the main issue - the kinds of encounters they are facing. Adjusting the encounters and altering the NPCs to meet the style of the PCs is something I myself would do. Again, as above if I gave the PCs a sidekick like a simple warrior they can all use (Squire Dyse Rolz) I'd likely stipulate they focus on the task at hand during combat and never talk or argue, unless really necessary - saving time to just rolls without much strategic thinking or distraction. I've played two full characters myself for a session - it was a bit of a mess for the second.
But to answer your specific question, if you wish to go down that route I recommend temporary items or weak items that are specifically planted for this kind of encounter.
But first of all, before providing these items I suggest that the PCs work for them so they don't take them for granted. For example, to defeat a particular enemy they hear has some particular weaknesses (or have run away from but still need to defeat) some roleplaying to convince a trader/weaponsmith/mage/retired adventurer to part with them for a PC-friendly price/reason would be in order. They do get to use these much more powerful items, but not without some character work on their part. A relatively quick task that the NPC can't do that we all know a party will take ages on (like riddles, amirite?) is good. Finding someone? Getting something rare from someone but has it in for the NPC? Something that might last maybe half a session of proper player work and/or rolls, unless it's more fun.
It also avoids cheapening the experience - something you had to work hard for is cherished more than something you just get handed to you, etc etc.
The Whatnots of Gifticality
Potions of Health etc are an obvious choice for survivability, but a higher grade, such as a Potion of Superior Healing. 8d4+8 means an average heal of 28hp, probably higher than the hitpoints of the PCs in this case. The upside is that it only requires one action to heal, rather than two or more actions for lower-grade potions. Saving an action next round for something else can be quite important in some encounters. Other options such as a Potion of Fire Breath, or variant type that isn't that enemy's particular resistance or attacks a weakness - Cold Breath, Acid Breath, etc. (Force resistance is quite rare in enemies too, incidentally. "Look at me, I'm Black Bolt!")
Another option are scrolls - one-time higher-level spell usage can hurt an enemy without leaving a super-weapon in the player's hands. Bards and druids can't save scrolls for later spellbook entry like a wizard can, if you're concerned they might save it.
Continuing the theme, altering an existing magic item to self-destruct can work too. Frostreavers from the Dragonlance setting are magical axes made of very specific ice from locations only known to a few makers in the far, far antarctic south, annointed with thanoi oil etc. It only remains powerful in cold or extremely cold environments, otherwise it's a rather weak magical weapon. The adventures in such a place were very rare, of course. It might be a fun character aspect to have a +1 axe made of ice that never melted, unless the DM judged that it would deteriorate quite quickly. A weapon bonus blessing gifted by a demi-god that lasted a single night also features in one of the Dragonlance novels, but again that would be a little deux ex machina if just given to the PCs.
Dark Sun (a resource-poor setting based on ancient Sumeria/Assyria) featured regular weapons made of bone, obsidian etc instead of steel, which lower-level PCs were more likely to be able to afford/find. They were lighter, had attack roll penalties and had a 5% chance of breaking when doing maximum damage, because they weren't particularly sturdy compared to steel. On this theme, magical items from a particular traveling weaponsmith they are unlikely to encounter again (unless you want them to) can warn them that due to the enchantments and crafting processes of the weapons they can receive they will only last three days (or a random period following the encounter) before they are unusable. And don't forget they could also access other types of items - a Broom of Flying that easily loses straw because it flies so fast, or a Circlet of Over-Blasting that starts to burn the user and eventually sets itself on fire after seeing too many dawns.
The key is that generally they won't last any longer than you want in case the PCs try to save them for a (much more) rainy day. You want them to use them now, so anything they get lasts for this coming encounter, maybe a little longer, but afterwards you're back to normal programming having some time to adjust your game a little.
Incidentally, my style is that if I used the temporary powerful item model I would ensure they could keep a small part of the item as a keepsake, given the work they put into gaining them. The slagged metal core of a burned-out fire mace's head, or the cracked central gem of a Circlet of Over-Blasting they could later put into a ring or a pendant. This was an interesting period of time that meant something and provided an in-character reward, rather than a DM-mandated fix to get over some maths for a session. Also you might get away with having planned it all along, which of course all DMs do, obviously, totes, because of course we did.
