DISCLAIMER- I don't own the creations of Rowling.
" You've got the brains of a Death Eater"- Aberforth Dumbledore
The Death Eater intelligence about the resistance at Hogwarts was completely wrong. They themselves knew that their Lord was on some pursuit, Bellatrix the trusted second-in-command was in charge of action.
The Carrows were in charge along with Snape, the Hogwarts teachers were quite submissive and Snape seemed to rule with an iron hand. Some twenty-five foolhardy students were holed up, led by two Gryffindors- Longbottom and Weasley, while amongst the Slytherins, Crabbe and Goyle became the favourite students of the Carrows- typical Death Eater muscle.
Bellatrix had known that the castle had no defences, except the enchantments resisting portkeys, apparition and flying because those can hit both ways. The floo network was disconnected, snatchers and dementors patrolled outside. She was not as dangerous as her Lord, but had enough skills anyway.
Just as the battle began, all calculations proved wrong. The Carrows were somehow knocked out, Snape had to escape and the juniors were somehow inactive. But even as the Lord was not fighting with them, Bellatrix had a few hundred fighters, around 150 dementors and few giants.
Giants have a problem, they cannot be transported magically and need much food. After the giants would wreak havoc and the dementors weaken the defenders and maybe suck out a few souls, it would be a cakewalk- the Death eaters assumed.
Just as the attack would start, the defenders sprung into action. Two brooms quietly flew out the Astronomy tower, each carrying two people, all disillusioned. They silently flew towards the two giants approaching from the lakeside. Demelza Robbins was carrying Ginny Weasley, Padma her twin. Both Ginny and Parvati were excellent with the reductor curse. Though giants are tough, a well-aimed reductor curse at their noses create the effect of a hard punch.
The curses were so powerful that the giants lost a chunk of their loses and unable to withstand the pain, lost consciousness. They fell to the ground, the slanting ground promptly rolling them towards the lake, where the giant squid quickly dragged them down. The brooms did not stop. They flew towards the other four giants, then hit two of them with confounding charms straight at the eyes. The confused giants swung their clubs wildly, hitting their kin and causing some of the Death eaters to apparate away in panic.
By the time the confusion stopped and anti-disillusion charms fired at the sky, the two brooms had flown back to safety and the four girls joined the defenders, who were doing their parts in the long thought-out plans.
The giants tried to knock down the walls with the clubs, only to leap back in pain. The castle walls were protected with momentum-reversing spells that resulted in the force reverting to their hands, causing the wrists of the giants to snap. The dark wizards managed to lift the spells off the walls and repair the giant's bones, but it took time. Finally, the walls were knocked down, the giants were running up the slanting grounds, the dementors gliding in groups while the Death Eaters sent a stream of reductor curses towards the castle.
There was no werewolf except Fenrir Greyback present, but it was anyway useless as it was not a full-moon night. The Wolfsbane potion may help the drinkers to keep their minds, but not morph.
The attack did not fare as expected. The castle was protected with with a honeycomb-like protective shield that clung closely to the walls, few parts of it were broken, the rest of the curses fell on the ground to create small holes.
The giants tried to run headlong, but as soon as they approached close, multiple things happened. An enchanted earthquake was created by runes inscribed on the obelisk-like stones in the grounds, causing powerful shockwaves that caused even the Death Eaters to fall to the ground. The giants were stopped for a few moments, but by that, thick tentacles formed out of the earth and pulled the giants down. Giants were strongly built, but not as much as hyped, they stood about 4-5 times higher than human males and not invincible.
The dementor attack was futile. They were gliding over the grounds, thus unaffected by the earthquake. They were hungry for battle, sensing dozens of human minds as their targets. However, as they came near, fifty corporeal Patronuses burst forth from the upper windows and castle peripheries.
While a patronus can protect the caster from dementors, a single patronus can charge straight into a dementor, causing both to be destroyed. While the caster can cast another, they are only mentally exhausted, the dementor is destroyed. Most powerful wizards can use a single patronus to destroy multiple dementors. As the dementors lost a third of their number in a single blow, they panicked and fled.
Bellatrix screeched "Lucius, stop them, round them up. Oh no, help the giants, you fools. Shoot whatever you see."
Unknown to the attackers, the patronus casters were drinking a magic potion that would instantly replenish their mental energy reserves.
As the giants fell to the ground, they tried to tear apart the tentacles. The move was calculated. Grawp the giant came running from another side, but he was not needed. The hundreds of animated armour suits and statues of Hogwarts, from the winged boars at the gates to the gargoyle of headmasters office suddenly appeared out of thin air (obviously by some magic) and began to viciously attack the giants. Grawp took a club from one and tried to hit them. In the end, two of them were killed while another two barely ran away from the grounds in panic.
The Death Eaters tried to help the giants by casting reductor curses directed at the animated statues, but it was not easy. First, jinxes were now coming from the castle peripheries and upper windows in torrents, they had to hastily put up shields.
Bellatrix shrieked again "Cissy, you stop the giants and tell them to wait in the clearing. Forward, you idiots. Hit anything that moves. No mercy, kill or blast anything that comes in front. Need I tell you to be on guard?"
The Death Eaters now directly walked to the castle ground, with silvery shields up in front and air-cushion charms beneath feet, precautions against what tricks have already been played. They began to rain reductor curses indiscriminately, causing the protective shields to glow yellow and vanish, then fragments appeared in the outer walls, shaking the castle. The statues had pulled back, the defenders were quiet.
The Death Eaters were happy, they felt that the defenders had retreated sensing defeat.
However, as they poured to the grounds, the next layers of nasty traps were activated.
As large numbers poured to the slanting ground, two things happened one after another. The ground shook again, but no earthquake this time. The ground began to reshape, the smooth lawn changed to a treacherous field of boulders, spikes and holes. The attackers were prepared, so quickly performed self-levitation spells but then the second trap activated. A powerful tornado was formed in the courtyard and it began to flung the attackers mercilessly towards the Whomping willow, whose magically strengthened branches simply beat the unfortunate bodies to pulp.
Some quick-thinking men did perform the countercharm to the tornado and set the tree on fire, but it had done serious damage by then.
Bellatrix looked at the statues standing quietly and had a brainwave. She shouted "shatter the rune-stones, they may have more inscribed spells. Ignore the statues, they cannot hit us from far. After the stones, send spells at the windows and in between the statues, the wand-carriers are hidden disillusioned there, I can bet my life on it. They are more danger to us right now."
The duelists were waiting tensely. There was another set of spells proposed to be inscribed on stones, which would summon tsunami-like waves from the lake and hit the grounds, then spawn thunderstorms, but turned down as it would endanger the merpeople and the squid.
Now, direct combat would begin