It's not likely Dittto had a huge HP ammount, almost any strong attack + hazard recoil will leave you below 25%, and more if your mere intention is use Ditto as just, and only Revenge Killer. And use Scarf Ditto as Shell Smash Bait is piece of cake, it just depends the moves you had and he copied, if you're, for example, copying a Pokémon who can't make nothing more than resisted hits on your Shell Smash User, he's done. If he's copying your Pokémon, K.O'ed one of yours, and you know you're totally able to resist that move on specific, he can't just do nothing except give you a wonderful Shell Smash Sweep. Well, the same can be said of any Choice-Scarf user, but with Ditto YOU KNOW he'll have Choice-Scarf, and even worst, you know what moves he had, so, it's the epitome of piece of cake prediction and free turns winning.
And I actually winned more games with balanced Ditto than only-revenge-killer Ditto, it's just great to fill roles on your teams taking a member of your opponent, need Stealth Rock? Just copy him's Stealth Rock user, and you'll have an extra move on your original Stealth Rock user (and you know he'll have a Stealth Rock user), need Rapid Spin? Copy him's Rapid Spinner. Need to Wish someone? Copy Alomomola, granted, Ditto's Wishes are not the biggest ones, but you're still recovering 35%+ of the HP of any Pokémon, and by almost free. Need to cleric your team? Copy the cleric of your opponent, stall him a while, and when your PP is about to end, you use Aromatherapy/Hell Beal and switch.
I finded of a lot of help a Ditto who's not choiced to something. LO Ditto is not very good, by other hand, Ditto will usually change a lot on mid match, and LO will destroy any durability it has. Again, the Revenge Killer function is no the unique function of Ditto, it can totally disturb a enemy team with other items (other good option is Red Card Ditto, if you're aiming for the sweep, just copy him's faster Pokémon who can't K.O itself on one round, he'll be forced to switch, and you'll have a free turn Set-UP or just to wreak havoc). Of course, any of those sets doesn't destroy at 200% him's potential as Revenge Killer, you're still having 50% chance of winning it if you're on a emergency, and if your opponent is smart, you can predict him and also win a free turn to wreak havoc.
Also, the outcome of the Tangela VS is pretty strange, it mostly depends on who sleep before and Sludge Bomb, of course, it also wins the one who have already a sleeped Pokémon, and if both have a sleeped Pokémon... well, nothing to do at all, just switch to your favorite steel-type Pokémon because he'lll Sludge Bomb you, or you can try Sludge Bomb him, you'll still have a decent chance to survive the attack and leave Tangela prone to being revenge killed, and if you copied the offensive set, well, just change to whatever who doesn't care about him's attack, the selection is huge.