Trevenant with Harvest and a Sitrus Berry can use it twice in a row minimum. First turn Curse, activate Sitrus Berry, potentially harvest berry, use again, eat berry, maybe Harvest again. If the EVs are divided correctly, it will have 51%, so it could shoot off another one. One fourth of your HP at the end of every turn is a pretty big deal, really limits what you can do, especially when your maximum recovery is 1/8th with Lefties and Ingrain. If used early on, you essentially limit the boosts your opponent can accumulate significantly. On top of that, if you can stall with Sturdy, Focus Sash, Protect or something similar, you can prevent them from sweeping you. Then again, if they play smart, they can just almost lose Espeon but then pass to Sylveon. Either way, they become significantly pressured. However, they most certainly can still win unless they get too greedy or are against a team of Sturdymons and Sash users where everyone has Protect.