Illusions and traps FTW
I recently ran a dungeon where the characters were up against a mad illusionist who had inhabited a mine and was living off of whatever adventurers dropped when they investigated the mine and died. It came off as pretty successful as it did the following:
- Players reported they had a fun time!
- I was able to deliver some new twists on old tropes like a trapped dungeon.
- Due to how I delivered the dungeon (roll20.net) I was able to use the environment to give the players a sense of what their characters were experiencing with illusions confusing their senses (see the note on the Seeming spell below).
Your mage might not be mad, but how sane can they be if they're trusting those little back-stabbing Kobold jerks to not betray them?
Following the guidelines set up for PCs
Hopefully your Kobolds can act somewhat independently. They've got 24 hours to enact a setup worthy of Tucker's Kobolds. To make sure your mage gets to take the credit for their work they're going to add as many illusions as they can to give players an even harder time of finding, disarming, and surviving the various traps that will be left behind.
Seeming (5th level) lasts for 8 hours and doesn't require concentration.
This spell allows you to change the appearance of any number of creatures that you can see within range. You give each target you choose a new, illusory appearance.
Since it doesn't change the toughness of your Kobolds you'll want to make them look like something the adventurers will avoid at all costs, like Brown Mold, Rust Monsters, and anything else that's a pain to defeat. Even better if those Kobolds are guarding the easy path you take back and forth in your dungeon, prompting players to take the trapped paths that look safer.
Alternately, if your Kobolds can wrangle up some monsters that are actually deadly to the players, you can make them seem like something easily defeatable (within the size restrictions set up by the spell).
In the dungeon I ran my illusionist had struck a bargain with Shadow Demons, not Kobolds, so this may be a little different for you, but when I ran this I had my illusionist cast this twice. The first time was right before the characters walked down steps to the next dungeon, making each character look like a different monster. This was initially confusing, but since it didn't have any immediate harmful effects they let it slide. The next casting was on a bunch of Shadow Demons on the next level, making them look identical to the characters. Seeing as we were using roll20 and since the Shadow Demons have incorporeal movement (allowing them to pass through enemy spaces) the players had a hard time telling friend from foe, which was the point. After a few times when they targeted each other (and fully enjoyed the sensation of themselves not knowing who was who, as opposed to having to separate character knowledge from player knowledge concerning the illusions), the players realized they had to really focus and keep track of which monsters were the enemy.
Hallucinatory Terrain (4th level) lasts for 24 hours and makes
...natural terrain in a 150-foot cube in range look, sound, and smell like some other sort of natural terrain.
This might be open to interpretation, but I'd say the natural terrain of a sinkhole-filled cavern could be made to look like the the natural terrain of a cave with a well-worn, completely safe floor.
In my dungeon the mad illusionist had disguised some drop-offs as being solid and some solid floors as drop-offs in order to try to lure the characters (who lack Feather Fall) to their doom. Luckily for my players (who often seem to do things the hard way) they decided to climb along a wall to avoid a hole in the ground (which in reality was solid ground) and they accidentally avoided the seemingly-solid hole, so I can't say for sure what would have happened if they'd gone the wrong way. However, in the event that they did go over the trapped ground I was prepared to offer saving throws first to realize they were about to fall and second to allow players on solid earth to grab ahold of the falling player. This was to emphasize the untrustworthiness of the dungeon as opposed to actually killing anyone off.
Major Image (3rd level) is short at 10-minutes and requires concentration, but is probably helpful to hold in reserve as a final way of covering up your ultimate trap of doom once the adventurers have dispelled your other illusions and regained their confidence. Especially if you can tie them up for an hour after they cast True Sight.
In my dungeon this was necessary to obscure the best trap of all; a plate in the ground that dropped a boulder on the mine cart track they were following so that it rolled towards them in an homage to Raiders of the Lost Ark. The at least nominally-vigilant group was looking for traps instead of feeling ahead for one and was surprised to suddenly feel their foot descending and watch it disappear below the illusion.
Pushing the boundaries of what NPCs can do
Since you said you'd prefer your wizard to "utilise the same or similar tools" you might be open to a little house-ruling. If so, you might decide that the NPC knows a custom version of Major Image that becomes permanent after multiple castings. Major Image already allows the illusion to become permanent if you cast it with a 6th-level spell slot, so you might find this isn't too far of a reach for you. Permanent illusions can cover up all sorts of pressure plates, trip wires, and anything else that triggers your traps.
Finally, looking ahead to the future.
If your mage survives this encounter (even if the PCs rob them blind) they might level up by killing off all the other adventurers who will also visit the dungeon. If they can reach a high enough level to get access to 6th and 7th level spells then you have a few more options. You'll certainly be casting Major Image with a 6th level spell slot well before the adventurers arrive until they won't know what's real and what isn't.
You'll also want to look into Mirage Arcane (7th level) which lets you
...make terrain in an area up to 1 mile square look, sound, smell, and even feel like some other sort of terrain. (emphasis mine)
Now your players have even less reason to trust their senses. Even better, the next time the players think they've found the treasure hoard what they've really found is a large pile of stones that look like gold pieces.
I absolutely did this with my illusionist's treasure hoard (and hopefully my players don't know my stack exchange user name). Since all of their gold went into a bag of holding they haven't yet realized that it's not chock full of worthless rocks, but knowing my group I think it'll be a source of hilarity instead of angst. YMMV.
You might also invest in Simulacrum (7th level), because nothing says "don't come back to this dungeon" like the players thinking they've killed you off when in fact it was your double. This works best if your double can throw themselves into a pit right before they die to the players don't see the double turn into a pile of slush.
OTOH, I had my players see the simulacrum turn back to slush because I wanted them to know their new nemesis was still out there. Being a DM whose always having to make guesses at what random place they'll travel to next in the world, it's nice to have upped the chance that they'll return to a place I already have well-mapped for a bit of vengence.
