One of the most powerful dark wizards in the world of Harry Potter, Gellert Grindelwald is memorable not just for his incredible magical talents, but due to his genius mind, charisma, and tactical abilities. While armed with the Elder Wand, Grindelwald is a powerhouse even when he isn't using a weapon as evidenced by the Fantastic Beasts series. The only wizard capable of besting Grindelwald is Albus Dumbledore.
More details on their confrontation are expected to be revealed in the third installment of the series, The Secrets of Dumbledore. But the world of anime is packed with magical characters that would draw Gellert's attention. Some, he could defeat. Others would be a challenge, or would even beat him with far more ease than his former lover.
10 Would Defeat: Louise Wouldn't Get The Chance To Make Good Use Of Her Abilities (The Familiar Of Zero)
At the beginning of The Familiar of Zero, Louise de la Vallière is known at her school as Zero due to the fact that her spells always fail. She ends up causing explosions whenever she tries to cast something. Louise's familiar summoning spell doesn't go that well either, and she ends up summoning a human, Saito Hiraga.
The Familiar of Zero later revealed that her failure in magic is caused by her being a Void Mage. Her Void spell Explosion is incredibly destructive and can obliterate armies. Unfortunately, it takes time to cast. Louise wouldn't get the chance to use Explosion, as Grindelwald would easily defeat her before she could display her true power.
9 Would Lose To: Sadao Maou's Powers Are Fueled By Negative Emotions (The Devil Is A Part-Timer)
Originally the Demon King from Ente Isla, the protagonist of The Devil is a Part-Timer Satan Jacob is defeated by the Hero Emilia Justina. Chased away to Earth, Satan finds his powers have been limited and he is forced to take on the name of Sadao Maou. Sadao's magic is remarkable, as he has displayed powerful energy blasts, telekinesis, the ability to fly, mind control, and many other skills.
Ironically, while Sadao normally can't use his powers as he used to, he would be the worst possible opponent for Gellert to face. Sadao can channel negative emotions and use them as fuel for his magic, which would make someone like Gellert unable to defeat him.
8 Would Defeat: Tanya's Weapons Wouldn't Help Her Against The Elder Wand (Saga Of Tanya The Evil)
After being killed in the modern world, a regular Japanese salaryman is reincarnated as Tanya von Degurechaff. Recognized as a child with magical potential, Tanya joins the army and is an excellent fighter, particularly skilled at air combat. She is able to use gospel magic, channeling her innate abilities through her Elenium Type 95 Operation Orb and her rifle to create massive explosions. Tanya's weapons wouldn't hold a candle to the far more versatile and powerful Death Stick. Her instability would make her vulnerable when faced with Gellert's tactical mind.
7 Would Lose To: Rimuru Can Wipe Out Armies With Ease (Reincarnated As A Slime)
Originally a regular human, Rimiru from isekai anime That Time I Got Reincarnated As A Slime is reborn into a different world as a seemingly lowly and inferior creature. In reality, this particular slime's powers are far greater than they would seem. Rimiru can absorb the skills of any opponent he defeats and he's eventually capable of wiping out an army of ten thousand people with a single spell.
Megiddo provides him with the fuel he needs to ascend as a Demon Lord. Resistant to the elements and to magic, Rimuru is basically untouchable by anyone other than Demon Lords on his level. Gellert would sadly not have a chance.
6 Would Defeat: Alice's Mastery Over The Sacred Arts Would Not Be Enough (Sword Art Online)
One of the main characters in Sword Art Online's Alicization Arc, Kirito's friend Alice is very talented in the sacred arts from a very young age. When she is arrested for breaking a taboo, her memories are taken from her and she becomes Alice Synthesis Thirty. Alive is a deadly fighter, and her work with her mentor Bercouli hones both her magic and her swordsmanship. As powerful as Alice is, her sacred arts wouldn't be enough to defeat Gellert. Grindelwald's mastery over the elements would overpower her, and he could easily disarm her with a simple Expelliarmus.
5 Would Lose To: Ainz's Skills and Undead Nature Would Outmatch Gellert's Magic (Overlord)
Overlord's Ainz Ooal Gown, is the epitome of an overpowered character. A human player who finds himself trapped in the world of the game YGGDRASIL, he has a significant advantage over protagonists like Kirito. By the time this incident happens, he has already maxed out his level. Ainz's guild holds countless priceless artifacts and his skills and stats are on the level of a boss monster that is impossible to beat.
Being undead, Ainz would most likely be immune to spells like Avada Kedavra. In fact, he's weathered high-powered holy spells his species should have been vulnerable to. Grindelwald would most definitely be outmatched.
4 Would Defeat: Shiroe's Enchanting Just Wouldn't Be Enough (Log Horizon)
In the isekai anime Log Horizon, the socially awkward Shiroe finds himself transported into the world of the game Elder Tale. Having maxed out his character and being a brilliant strategist, Shiroe proves to be essential in the adventurers' quest to make sense of their new lives.
It's notable, however, that on the battlefield, Shiroe is an Enchanter-Scribe. That makes him primarily a support class. His direct combat skills are somewhat lackluster. His strategies and spells would fall apart when faced with the power of Gellert Grindelwald.
3 Would Lose To: Echidna's Magic Far Outclasses Grindelwald's (Re: Zero)
One of the supporting characters in Re: Zero, Echidna ranks among the antagonists of the anime but has nevertheless earned many fans of her own. From her very first meeting with protagonist Natsuki Subaru, it becomes obvious just how dangerous and powerful she truly is. Her power is so great that she is able to wield all six elements. Not only that but she is capable of creating life, and is the source of Emilia's protector Puck. While Grindelwald's cremation spells can be devastating, they couldn't even compare to Echidna's magic.
2 Would Defeat: Clara Is A Little Too Childish To Be A Challenge (Welcome to Demon School! Iruma-Kun)
One of the deuteragonists in Welcome to Demon School! Iruma-Kun, Clara is a good friend of protagonist Iruma Suzuki. She is a demon, and despite her eccentric personality, she has a few remarkable talents. She is able to summon or recreate any object, person, and item, pulling them out of her skirt pockets. She can even create a pseudo-illusionary realm where she can keep people captive and drain them of their magic. Despite the destructive potential of her ability, she is much too childish to be a real threat to Gellert. He wouldn't even have to fight her, as she would probably be very easy to manipulate.
1 Would Lose To: Lelouch vi Britannia's Geass Overpowers Gellert's Occlumency (Code Geass)
Even in terms of magical abilities, some powers are more unusual than others. In Code Geass, the power Lelouch receives from C.C. is too mysterious to properly define. Calling it magic would be perhaps inaccurate, but C.C. definitely calls herself a witch and Lelouch identifies as her warlock. Interestingly enough, Lelouch and Gellert are similar in some ways, as both are charismatic individuals who start deadly revolutions. Gellert's magic is far more versatile than Lelouch's, and upon first glance, he would defeat Lelouch.
But nothing is ever quite so simple when it comes to Geass. Lelouch's ability would easily overpower Gellert's occlumency. Gellert may be able to harm Lelouch in a face-to-face confrontation, but after having received a code, Lelouch is immortal and can heal from any injury. There's really not much Grindelwald can do against that.