This starts with the opening scene, when a sentry atop the Great Wall of China is surprised by what appears to be half the Hun army.
How is that possible? Don't they send out patrols north of the Wall to, you know, give them some advance warning of approaching massive armies? And it's not even just the grappling hooks; as we see in a second, the Huns are already inside the Wall itself (how is not explained).
The brave Chinese watch commander manages to get to the top of the tower and light a signal fire, spreading the word that the Huns have come to China. "Now all of China will know you are here," he says defiantly, and Hun leader Shan Yu replies, "Perfect".
"Perfect"? What, did he plan it this way? If so, why attack in the middle of the night when everyone is least prepared? If not, then why allow the guard to light the signal fire? What does he gain by losing the element of surprise? Later, Shan Yu explains his motivations to a hapless Chinese scout, saying that "By building his wall, [the Emperor] challenged my strength". This is presumably why he wants to fight the biggest Chinese army at a place and time of their choosing, to prove his strength, but that's a horrendous military strategy. Half of the point of war is not letting your enemy know where you are and what you're doing!
Anyway, that's just the first of many bad decisions. Shortly thereafter, Captain Shang's father, General Li, leaves to fight the Huns on a troop of white horses. Cavalry. He's a cavalry commander. So why in the name of Sun Tzu does he stop and wait at the Tung Shao pass for the Huns? On paper it's not an indefensible idea; if the mountains act as a natural barrier except for that pass, that would be a good place to stop the Huns. But they don't. When informed of this development, one of Shan Yu's aides says "We can avoid them easily". Therefore, there must be other routes to the Imperial City. Therefore, giving away the biggest thing any cavalry troop has going for it--mobility--in favor of sitting and waiting at the pass is stupid. They didn't even build any fortifications that we can see in the movie, choosing just to sit and wait in some village until the Huns crashed down on their heads.
Shan Yu, of course, doesn't pick the "avoid the enemy soldiers" option. He attacks them head-on with all his strength and wins. We're not given many details about the battle, so it's reasonable to assume that there was some kind of strategic reason for eliminating China's best troops right there, as opposed to bypassing them and moving on to Peking. Still, it's hard to believe that most military commanders would choose to fight the enemy's elite forces on ground of their choosing when an "easily" used option to avoid them exists.
The centerpiece of bad decision-making, however, is the movie's biggest military confrontation: the battle in the mountains. After discovering the rubble that used to be General Li, Captain Shang and his command head through the pass to the Imperial City, telling us that they're "the only hope for the Emperor now". (What, the whole rest of the Chinese army was twenty guys on horses?) They're on their way through when Mushu accidentally sets off a cannon and gives away their position, as Shang yells at Mulan.
The problem is, the Huns shouldn't have needed the cannon to see the Chinese! Look at this picture:
Or this one:
The Chinese army are moving dark figures on a field of pure white. Yes, they're in shadow, but they'd still be clearly visible against the snow. How did the Hun archers miss them? Especially since the only reason for them to still be in these godforsaken frozen mountains, let alone posted up on a peak overlooking the pass, is to be watching for Chinese reinforcements! If they're within bowshot of the Chinese party, it's not like they're out of sight, either. There's really no excuse for the Hun lookouts to miss the Chinese until the latter throw up a big, red, explosive "Hello I'm here, come shoot me!" sign. Also, Captain Shang just saw evidence that at the very least, a massive Hun army was here very recently. Why are the men in a straggling, disorganized line? At the very least, shouldn't they be prepared for battle?
But never mind that. After the arrows begin to fall, Shang gets the men to the safety of some rocks, where they reply with cannons. The cannons appear to wipe out the archers, and all is well. Then... THIS happens.
The entire Hun army shows up and attacks. Apparently they were just sitting in the mountains and waiting for Shang's reinforcements to arrive. There is no logical reason why they should do that. In the first place, Shan Yu wants to sack the Imperial City. Well, it's right over there. There's literally nothing standing in his way from here to there. Second, nobody in the Chinese army command knows Shang is coming; Mushu issued fake orders to get Shang's company (?) into the fight. Shan Yu couldn't have known they were coming even if he'd captured and tortured General Li. Third, even if you wanted to have a rear guard just in case, why leave your entire army there? Why not just leave a few hundred soldiers and move on? Fourth, when you see that there are less than 20 Chinese soldiers, why (again) attack them with your entire army? Why not just leave, say, 150 men to deal with this pathetic little band while you're off destroying Peking?
There are other reasons why this is a terrible idea from a tactical perspective. Here's another picture of the Hun charge:
That little black dot out in front is Shan Yu, who apparently wants to kill all the Chinese himself, which you don't want your general to attempt for obvious reasons. Look at that vast, snow-covered expanse, though. All of those Huns are charging down it, which means their horses are going to be at least tired when they reach the bottom, if not dead from falling down hidden pitfalls or crevasses in the snow. But more to the point, from the fact that they're charging and the fact that they're going downhill, it's going to be very hard for them to stop. This is bad because...
...they're charging towards a sheer cliff. And the people they're trying to kill are basically standing right on the edge of said sheer cliff. I don't blame Shang that much for picking this horrendous tactical location; it's understandable, given that he was trying to find cover from arrows at the time and that was the only available spot. That's relatively forgivable. But Shan Yu, once again, is charging towards a fucking cliff. And lest we forget his lesser sin, the people he's trying to kill are concealed amongst giant rocks, meaning that the horses will be relatively useless and might even break legs or whatnot if they, y'know, continue to charge straight at the Chinese and into the friggin' rocks. Rocks and a sheer cliff: yep, looks like charging down the mountain was a great decision.
And the thing is, from Shan Yu's vantage point, this is all completely known. He starts the battle probably one or two hundred feet higher, elevation-wise, than the Chinese do, so at the very least he should be able to see that the land abruptly ends. Plus, he's had the opportunity to survey the terrain, having been here first. Which essentially means he picked this spot and picked this excuse for a strategy in advance.