Based on the specifications of the Babylon 5 ships and weapons, and let's take say, a Minbari War Cruiser - The size being very large, just look at the flybys with B5 in the same shot - very big, and at least three times bigger than an Earth destroyer. Now taking all that into an account, and a Minbari ships fusion beam systems - Not sure how many, but has several smaller ones and the main cannon at the front. Look at the SIZE of that cannon when it fires, aka Freespace 2 big greens (though let's not get caught up in trying to compare them, Minbari beams are obviously ten times that of a GTVA one). Now if this ship is VERY big and the beam is VERY big, it must be VERY strong, eh? This is a simple way of looking at it, but that's how I see it. The Defiant's shields would deflect the opening shots, but with such a high-recharge rate and impressive size, the Defiant can't even take down a Dominion Battlecruiser - What chance does it have on a Warcruiser, or anything else that can DESTROY a warcruiser? And if a Shadow battlecrab can tear a Minbari War Cruiser in three-four shots, then I think it would take ten Defiants to take down a Battlecrab. Vorlon ships are, probably, equal to Shadow vessels as we've seen. It just depends on "who" gets the first shots in.
Oh, one more thing - Shadows don't have cloaking devices, they cannot "disappear", they simply jump out. I don't know the details, but it's basically an advanced form of jump drives.