This is tough to write as its not so clear-cut like it is with other decks as there's nothing that quite hates out Niv Mizzet other than Humility. However, there are some pods that spell trouble. As I mentioned in the "Why not to play" section, its hard or impossible to keep up with 3 fast combo decks. The worst matchup imo is the Frog Monster. Other combo decks if piloted well will not just jam their combo into your untapped blue lands (unless piloted poorly). They'll try and sculpt and prepare a hand that can combo after someone else has tried to go off or the conditions are right. This means they're in a sort of waiting game with other players, there's good back and forth happening in between, and it gives you time.
Gitrog on the other hand can keep on jamming things and doesn't run out of gas. The worst position you can be in is the table police, trying to stop the fast combo deck, while other folks are assembling combos and seeing you use your counterspells. Try and get others to use countermagic to stop them. Multiple Gitrogs is bad news. On this all I can say is, Cursed Totem can help, Gilded Drake can help, Pongify can help, stop the Frog, counter Necropotence and Ad Nauseum, and try your best.
Mixed pods can create very good conditions for NMP. When players have to be cautious or work around other stack pieces and things get drawn out longer is a good place to be. Sometimes its worth protecting another players stax pieces. Stranglehold, Rest in Peace, Rule of Law, Ashiok, etc can all work in your favor. I've found that one person running Blood Pod is excellent as we have good answers to their win conditions and they put extra pressure on other decks.
Individual Tactics:
Gitrog: Counter/destroy/Gilded Drake Gitrog if possible is the best tactic. Its the biggest speedbump for them. Be wary of Ad Nauseum or an early Necropotence. Often a good candidate to Git Probe early if you have it
Prossh: Prossh can be a difficult matchup like Gitrog. While Food Chain is their plan A, their plan B of beatdown is very good as well. Countering Prossh doesn't stop his army of 1/1s made more difficult with ETB or Blood Artist effects. They also don't have a lot of instant/sorcery interaction which is less draws for NivMizz. Be wary of Food Chain and try not to let their dudes get your health down too low. Stranglehold and boardwipes should be favored here.
Urza: This matchup has been fine overall. They're not the fastest deck ever either. Be aware of their stax pieces as Winter Orb or Static Orb can be trouble, so save your Salvage and Abrade for them if you have it (or as an emergency if they drop Scepter or Dox Engine).
Flash Hulk: All the decks stax pieces are strong here. If there are multiple decks on it, its probably worth taking a mulligan or two to try and get a stax piece, value them higher than normal. If you have a Fluster, use it well.
PST (Paradox Scepter T&T): This is a fairly good deck to be paired with. They're slower and more deliberate than many other combo decks. They have a high level of interaction which is good for us. It means you have an indirect ally to fight faster threats and pull the game out longer. Watch for open Ad Naus and Notion Thief.
Food Chain Sliver: Counter Food Chain if possible. The list is more dependent on dorks than others since they ramp after Food Chain as well, so value Pyroclasm effects slightly higher than normal. Necropotence is very good for them, and even more deserving of countering than normal. Be aware of Ad Naus
Tymna Kraum Thieves: A deck around Wheel + Thief shenanigans. This deck is dangerous, but not particularly fast, and also a bit more predictable since it relies on A+B wheel combos. Instant speed creature removal and bounce should be valued more in this matchup. Save them to respond to a wheel effect. For example if they play Notion Thief eot, allow it, then in response to Wheel on their turn bounce it or remove it. This isn't always correct based on how much value other players could get from the Wheel, but is generally what you want.
Selvala/Yisan: not much to say here other than wipes and Cursed Totem are good. Early NMP is good here if possible as the ping is really useful against them.
4c Rashmi/Curious Control: Our curious brother/sister deck. They get out curiosity dudes quicker than we do generally and have a slightly higher board wipe count, so keep that in mind. Spellskite is good here to redirect stuff. Note that Misdirecting or Spellskiting Curiosity won't give you the benefits, its still under their control. However, this means you can also piggy back of their curiosity dudes too. Putting Curiosity or Ophidian Eye on a Vial Smasher or Electrostatic Field is pretty funny. Note that you will only draw cards when it hits your opponents, not you.
Razakats: Just be aware of the combos. Removal and Counters are good, all your Stax pieces are good.
Consultation Kess/Storm: Another deck that can and often does run Cursed Totem is nice.
Gilded Drake can be good here as the game goes on, but don't make it your main focus as casting Kess isn't theirs. Don't let an Ad Naus resolve when their health is in a good spot. Attack them when possible with stuff like Treasure Nabber or Lannery to whittle them down a bit, ping them with Niv Mizzet when possible. Their life is very valuable to them and they don't run many blockers. Expect Tymnas to be swinging at them as well. Make sure you or someone is able to interact with a Consultation Jace out of nowhere
Zur: Haven't been able to test against any Zurs yet, just hasn't come up in pickup games. In general though try not to let them attack with Zur. Other than that storm lists play a lot like Kess but slightly worse. If you're unsure of the variant, be on the safe side and worry about Consultation Jace. Control variants are clunky, and you should be able to spot them if they get Rest In Peace or start dropping some more unusual enchantments. Be wary of Helm of Obedience if they pull out RiP
Korvold: Korvold has been a tricky one and more difficult to deal with than Prossh. He pairs even better with
Dockside Extortionist than Niv does, to the point where countering dockside if able is quite valid. Saccing 5+ treasures for mana and drawing into more combo pieces, or being able to reanimate him and continue the chain to build up things like
Veil of Summer or
Vexing Shusher in preparation for casting
Food Chain.
Stranglehold is good here as it shuts down a lot of their tutors.