Dark dugtrio + (neo 2) Fotoresu
This combo will make any opponent think twice about retreating.
Everybody should be familiar with dark dugtrio's pokemon power:
"sinkhole" Whenever your opponent's active pokemon retreats, your
opponent flips a coin. If tails, this power does 20 damage to
that pokemon.(don't apply weak. or resist.)This power stops
working while Dark dugtrio is asleep,confused,or paralyzed.
For those not familiar with Fotoresu here is what he does:

Fotoresu
Type: Stage 1 Steel Pokemon (Evolves from Kunegidama)
LV.40, HP 80
[Pokemon Power] (Makibishi)
  Effect: Whenever your opponent Retreats his or her Active
    Pokemon, this Power does 10 damage to that Pokemon.  This
    power stops working if this Pokemon is Asleep, Confused or
    Paralyzed.
SCC: High Speed Spin: 30
  Effect: If your opponent has any Benched Pokemon, he or she
    chooses 1 of them and switches it with the Defending Pokemon.
    If you have any Benched Pokemon, choose 1 of them and switch
    it with this Pokemon.  (Do the damage before switching
    Pokemon.)
-CCC Retreat
R Weakness
G Resistance
No. 205 (Rare Hologram)

Do the math...

one Dark dugtio (DD) = possible 20 damage
one (DD) = one Fotoresu (F) = possible 30 damage, guarenteed 10
two (DD) + one (F) = possible 50 damage, guarenteed 10
two (DD) + two (F) = possible 60 damage, guarenteed 20

You get the picture.

For more devistating effects here are a few cards to add to the
formula...

Dark Machoke: Drag off can bring out a pokemon of your choise
              and leave it stranded.

Gust of wind: same effect as Dark Machoke's Drag off except
              doesn't use up an attack.

Hitmonlee   : can pick off pokemon building up on the bench
              while loading up for high jump kick.

sneak attack: keep them from using scoop up or switch.

Guraigaa    : Scyther like fighting pokemon with poison
(translation) capabilities, it'll make anyone try to retreat
    below

Guraigaa
Type: Basic Fighting Pokemon
LV.31, HP 60
F: Poison Sting: 10
  Effect: Flip a coin.  If heads, the Defending Pokemon is now
    Poisoned.
CC: Slash: 20
(No) Retreat
G Weakness
F Resistance (Damage -30)
No. 207 (Common)

I'm still tweaking the deck so if you have any suggestions please
send them to mikes_spectrum@hotmail.com

*Thanks to Edo "the one and only source of translations"
& to pojo "the best source of tips & strategies"