I see the standard set being:
Leftovers
252def/252spdef
Leech seed
Encore
Substitute
Gale/uturn
Come in on any non damaging move and force out with encore. Proceed to subseed. If a grass type comes in to ruin your fun, hit him with a powerful gale. Specific offensive evs could be invested to ensure certain koes. Alternatley, you could run uturn over gale. It's less powerful and forces you out, but more accurate, let's you maim celebi, and is a powerful scouting tool, especially when combined with encore.
Erufunn definately suffers from Move slot syndrome and empty turn syndrome (See jumpluff), and I personally think all Eru's should run taunt (priority taunt makes you able to taunt taunters) with the last slot being a choice between Gale, u-turn, toxic or encore.
If you choose encore, running 252/252 def and sdef is a great idea since you'll be hitting everything with priority anyway, though you may want to maximise speed for gale and u-turn and split between the defenses. Your last slot pretty much determines who shuts you down.
If you go for encore, Espeon is going to school you with magic mirror, and you won't be able to do much against rival grass types bar shutting them down, if you pick gale or u-turn, you'll lose the ability to switch in on boosters to encore them and bulkier grass types (and still, espeon) will give you trouble.
If you pick toxic, your best counters are Grass/poisons such as venusaur and roserade as well as nuttre (who pretty much walls 90% of Dppthgss OU).
All these pokemon are destroyed by shanderra, making erufuun and shanderra one of the most annoying combos in BW (Theorymonning here, of course), Erufuun stalls and shuts down the opponent, while shanderra traps and eliminates his counters.
EDIT: Ideally, the two erufuun sets I see are;
Erufuun@Leftovers
Bold
252 Def/252 SpD/6 HP
Leech seed
Toxic/Encore
Taunt
Substitute
Or
Erufuun@Leftovers
Jolly/Timid
252 Spe/126 Def/132 Sdef
Leech seed
Gale/U-turn
Taunt
Substitute
Offensive Erufuun is a stupid idea, It's more of a gimmicky set I can't see making any headway in OU so I'd take that set out the OP. Key to it's strengths and work with it's priority stall/subseed powers.