After alien Santa's army was wiped out, he casually took the field alone and demolished half of Halloween.
Shogun from Harkovast kills what is literally a small army of Nameless warriors. To make matters worse, about half way through he pauses to announce that (despite having him ridiculously outnumbered) THEY are the ones who are all going to die.
Skoll of Cry Havoc has a body count of a paltry fourteen. Take note however that four of those kills were with a sword against assault rifles and shotguns. Three more were the crew of a main battle tank. That she charged down. As it fired every weapon it had. And hit her. Suffice to say, she worked for those kills.
In Super Temps that is main character Skully in a nutshell. Her powers allow her to decimate any threat so well that she has to hold herself back. If she went off the deep-end at any point, mass destruction and chaos would ensue!
The big ones, though, are Roy, V, and Belkar. Between strips 917 and 924, the three, in rapid succession, face down an entire army. First Roy, effectively alone, with hundreds of arrows raining down upon him, as well as thousands of incoming soldiers, proceeds to speak confidently about surviving for hours until dusk- and then starts making good on that statement. As he's slowly overrun, though, V comes back from Hell] sets up a Wall of Fire around the protagonists, putting the attacking army on the defensive. Then, as the fires begin to lie low, Belkar hijacks an Allosaurus and proceeds to rip the entire army to shreds with it.
Karnak in Dominic Deegan by necessity. It's noted he doesn't really seem to gain or keep followers very well, so any battles he ends up fighting, he pretty much has to do it on his own. Being the single most powerful demon in Hell by this point certainly helps.
In one of the latest arcs, we see a battle. On one side, Demon Siegfried and a gigantic battalion of demons. On the other hand, Karnak. The tagline? Two armies clashing.
Pick any member of the main cast in Chaos Theory. Each of them can dish out epic destruction in their own way.
Connor hacks into a security system to make people get killed by their own defenses.
Kevin drops explosives everywhere, leaving mostly rubble and singe marks.
Troy literally punches out tanks and wields an AA gun like a pistol.
Almost every single notable character in Homestuck; however Jack Noir takes the cake, who lights a planet on FIRE, and casually destroys at least 15 more as well as later destroying an entire universe from the outside.
Several characters in Tower of God, especially Zahard's Princesses like Yuri and Maschenny and the High Rankers like Yu Han-Sung, Koon Eduan, Evankell and Zahard himself. For them it is already at the point of subversion because what they are able to do has reached a terrifying notoriety.
Then there is the case of Irregulars, those whose presence violates the basic rules of the Tower. Mostly because they were able to open the doors to the Tower themselves, they are among the most powerful beings in the universe, so powerful in fact, that they are either the highest ranked Rankers or go even above that. Urek Mazino was able to climb the Tower in less than fifty years, at least 10 times as fast as anybody else. Enryu killed the Guardian of the 43rd floor, even though they were practically immortal. And finally, Phantaminum appeared out of nowhere and stormed Zahard's castle, killing hundreds of High Rankers in his way, reaching Zahard and decided, for some unknown reason, to leave again. In the end, there was no choice but to make him the highest ranked of the Tower.
On a smaller scale, Ja Wangnan convinced Baam to make a truce with him because even if he was strong, he wouldn't hold out against the next six contestants. When the rest arrive and start squabbling, Baam decides to fight them all and nearly beats all but Ja, whose master skills and PokéBombs save his ass.
Jacob Doe in Genocide Man has taken on large groups of both wannabe bioterrorists and professional soldiers with nothing but his augmentations and a handgun (granted a full-auto railgun with sarin-tipped darts). It's said that Genocide Men are designed to take on armies.