Juggernauts are very powerful, but their greatest strength is that they require an approach completely different from the one you need against other troops. In a large fight - many juggernauts vs many troops, the juggernauts will usually win, unless the troops are high initiative archers. You simply cannot stand and fight them toe to toe.
But juggernauts are vulnerable to hit an run tactics, at least when there's only a few on the battlefield, i.e. use high initiative troops to dash in, hit them, and dash out between the juggernaut turns. You can make this easier if you have a water mage to slow them down.
Blindness can also work well against them. Use high dodge troops, and even the back-swing will be useless.
You can also try to get them to kill each other - give them a target, but make sure that said target is close to 2-3 juggernauts.
Then, you can also look at the specific kind of juggernaut you are facing, i.e. their traits. Unless you are playing at very hard difficulty, the juggernauts cannot, at the same time, be fast, and strong, and have lotsa HPs.
There's many ways to kill juggernauts, the problem is if you have to fight them with troops designed with other enemies in mind. Heavy armor for example is great in general, but against juggernauts it's only good at slowing you down.
But all this is just talk. If you look at specific ways to deal with juggernauts, you can check some of my play-throughs. The next to last involved killing a ton of juggernauts. It was in .982, so a few things have changed, but the approach outlined there works quite well against everything. At some point, I had governor heroes solo multiple juggernauts in that game.