There's so much wrong with this I don't know where to start.
I'll admit. I haven't seen the continuation to this. I am, however, going to extrapolate based on the characters' reactions and descriptions of this "brilliant" tactic.
There are many reasons that this is not just stupid, but painfully stupid.
1. Look at that terrain. Even on skids and being pushed by Slayers, that torpedo should have all the speed and accuracy of a hippopotamus on ice. It hits one crater or bump and it's going to go off the mark, or tip over.
2. Armour does not bunch up like that, unless they absolutely have to, like passing through a chokepoint or a city street. It's convenient to let one blast tank out multiple vehicles, true... but that is precisely why they don't do it.
3. Real military forces aren't like RPG AIs. They don't attack along one vector and nowhere else. The Slayers' home is out in the open. Even provided using this torpedo takes out this one force, they've still got the attackers on the other 350 degrees of their base to worry about. Or at least, they should.
4. Aircraft. Even if you don't want to use jets for whatever reason, no modern mechanized push goes through without gunship support if they can help it. The monastery is a helicopter's wet dream. Unprotected targets all clustered together without any anti-aircraft defences (I assume) mean that one pass would be enough to obliterate the Slayer Army.
5. Torpedoes work on a contact/proximity fuse. For example, there's a lot of ballyhoo about the threat that antiship mines pose to sealife. Mine contact fuses are specifically designed not to detonate like that. Wave action will not set off a mine. A dolphin or whale brushing up against it will not set off a mine. A destroyer, freighter or other large, heavy object will. You want your torpedoes to go off as close to your target as possible, not set off from water pressure or if it smacks into something en route.
By the same token, a torpedo's proximity fuse is utterly useless, because it either relies on acoustic, pressure waves, or magnetic sensors to detonate.
A tank does not emit the same sounds as a submarine or ship under water.
This is air, not water. No significant pressure waves.
Submarine metal content >> tank metal content.
Unless Xander has tinkered with the fuse mechanism. In which case, he's a damn idiot.
That being said, Buffy thinking that using werewolves against tanks was a good idea is absolutely hilarious.