He uses the sceptre, of course

Loki paced nervously on the balcony, awaiting his brother's arrival. Above him, the Tesseract's sky beam was rifting a schism in the fabric of reality, opening a portal to allow his army to come pouring through. The pathetic humans were of no concern, but Thor... he was far more worrisome. His mind clear, calmed by his successful recruitment of Iron Man, Loki knew he could not hope to best his brother in a straight fight. He must use his two best weapons - the sceptre, and deception.

"Loki!" The frost giant heard his brother's voice from afar, before the god of thunder landed with a thump, cratering the balcony. "Turn off the Tesseract or I'll destroy it!"

"You can't," laughed Loki. "There is no stopping it. There is only war."

"So be it," growled Thor. He charged at Loki, who swiftly sidestepped, allowing his brother to run past. Thor grunted angrily, then charged again, spinning his hammer as he did so. Loki quickly jumped out of the way, running into the Stark penthouse. Thor roared as he followed, too quickly for Loki to dodge... however, just as Mjolnir should have impacted with Loki's chest, it met only air, and Thor stumbled off-balance through the illusion. The bulky god suddenly found himself careening towards a wall, and barely skidded to a stop in time. When he did so, great steel restraints extended from all sides to clamp him against the wall, leaving him struggling against them.

Loki dropped his illusions, laughing triumphantly. "Ah, brother. I see you've run into a present from my new friend." Iron Man had built the restraints for Banner, so they should hold Thor for long enough. "You're so strong, brother. In body and in mind. But even gods..." he pressed the tip of the sceptre to Thor's head, "...can fall."

Thor gritted his teeth, then let out a long roar, as he battled the Mind Stone's advancing tendrils. Loki licked his lips excitedly, sensing his brother's admittedly powerful defences being inevitably eroded away. However, as he heard a screeching of metal, the frost giant flicked his black eyes down to Thor's arms, which were straining against the metal bonds. As the steel began to bend and deform from his mighty strength, Loki's eyes widened, and he pushed the sceptre harder against Thor's skin, willing the Mind Stone on with all his mind.

The right arm clasp tore from the reinforced wall, and Thor seized his brother's arm, trying to wrench the sceptre away. Straining against the bigger god, Loki just managed to keep the tip on Thor's head, only able to resist because the Mind Stone had already weakened him so much. Finally, Thor's yelling reached a climax, and he released the sceptre as his eyes closed in agony. When they reopened, they were a glossy black, which cleared as Thor blinks.

Loki's face was split wide by his smile, as he saw the telltale blue glow in the Asgardian's eyes. "Do you feel better now, brother?"

"Much," answered Thor. "The humans will not stand a chance against us."

He began spinning Mjolnir, preparing to fly, but Loki interrupted him. "Try to keep them alive, would you? I quite like the look of that sexy assassin girl."

Thor nodded, then launched into the fray.