name: Special Trick Room Sweeper
move 1: Trick Room
move 2: Hidden Power Fighting
move 3: Ice Beam
item: Oran Berry / Focus Sash / Life Orb
evs: 120 Def / 240 SpA / 80 SpD
ivs: 30 Def / 30 SpA / 30 SpD / 2 Spe
With 8 Speed not much will go before this under Trick Room, and if it gets a Download boost not much can stand in the way of its awesome 28 Special Attack. This can sweep teams without a Special wall if you get Trick Room going, and with 23 HP and 15 and 16 Defence and Special Defence respectively, it's not that hard.
Moves are pretty straight forward BoltBeam is nice for type coverage and Hidden Power [Fighting] takes down the main Special wall, Munchlax, although Shadow Ball could work over it though.
Oran gives you a second chance to set up Trick Room against weaker attacks, while Focus Sash Guarantees your first set-up even on the strongest opponents as Trick Room will always go last and you will most likely take a hit if your opponent doesn't switch. Life Orb can also work if you want that extra bit of power, but the recoil is a waste of those great defences.SET 2
name: Choice Specs
move 1: Ice Beam
move 2: Thunderbolt
move 3: Hidden Power Fighting / Hidden Power Ground
move 4: Shadow Ball
item: Choice Specs
evs: 116 Def / 236 SpA / 156 SpD
Even without a STAB attack this thing can be a major threat due to its brilliant type coverage and sheer power (Choice Specs+Download = 42 SpA, even without Download it still hits 28) Hidden Power [Fighting] can take out those Munchlax who seek to slow you down, while Ground beats down the BoltBeam resisters, namely Chinchou and Magmemite.
No EVs have been invested in Speed as the max without a nature boost is 13 and this does not outspeed anything worthwhile so defensive EVs are more useful.SET 3
name: Mixed Trick Room Sweeper
move 1: Trick Room
move 2: Thunderbolt
move 3: Return
move 4: Ice Beam
item: Life Orb / Focus sash
evs: 196 Att / 36 Def / 236 SpA
ivs: 0 spe
This set really takes advantage of Download, and can take out many of the Pokemon that stop the pure Sp. Sweeper. Shadow Ball is an Option over Thunderbolt or Ice Beam for those Ghosts.OTHER OPTIONS
Choice Scarf could work, but even at max speed and a + Nature it only reaches 21 speed, a number considered mediocre by Choice Scarf standards, so Specs is often a better option. If you do use Choice Scarf max out speed and use a Timid nature.
Porygon has a few options not mentioned on any of the sets, Hidden Power Fire / Fighting / Ground, Psychic, and Charge Beam can all work, but they do not significantly improve type coverage. If you are desperate for STAB your only special options are Hyper Beam, which, even though its side effect is horrible -- especially in the fast paced Little Cup -- has massive power, or Swift which has so low base power that it's almost not worth using, even with STAB.
Recycle and Oran Berry could let you use a defensive set to some effect, however Shoddy Battle has not yet implemented Recycle so it can't be used yet.
Porygon's other ability; Trace has some use for revenge killing trappers like Diglett, Magnemite and Trapinch, and for Tracing an opponent's Flash Fire/Volt Absorb/Water Absorb to get a easy switch in and possibly free healing off and immunity to their STAB attacks. However the ~50% chance of a Special Attack boost is generally preferable as on most sets it opens up a chance to sweep. If your team has problems with those Pokemon with the aforementioned traits then Trace can work very well.ILLEAGAL MOVE
Sadly one of its best moves; Tri Attack is illegal at Level 5 but it is worth noting if you play Little Cup Level 100 (it learns it at Level 51) or don't use the Level 5 moveset rule. The same goes for Agility (learnt at Level 12) which would be a nice alternative to Trick Room and Recover (learnt at Level 18) could let a defensive set become viable. Magic Coat (learnt at Level 56) could reflect back Sleep and other status moves. Other notable moves that are not legal at Level 5 are Discharge, Signal Beam and Magnet Rise.EVs
236 EVs maxes out Porygon's Sp. Attack, leftover EVs are put into Defenses. For Trick Room sets you want to have a 0 Speed IV and a - Nature. For the Mixed sweeper 196 maxes out Attack. For the Choice Specs set, should you choose Hidden Power [Ground] you should use an EV spread of 116 Def / 240 SpA / 80 SpD / 36 Spe along with 30 IVs in Special Attack and Special Defence.OPINION
Decent stats were it needs them, a excellent ability, great movepool and some degree of unpredictability; Porygon would have the makings of a Little Cup standard if it could just use Tri Attack but even without STAB it is a serious threat. Its other problem is its speed, it needs to set up Trick Room to be much use at sweeping, but once its set up it can be hard to stop it.COUNTERS
Munchlax or Bronzor can stop it (and outspeed in Trick Room) if Download did not boost Special attack, however mixed versions can beat Munchlax. Chinchou does nicely resisting BoltBeam and having fine defences to take even Download boosted attacks.
Trapinch can revenge kill it under Trick Room. Bulky fighting types (or bulky Pokemon in general who don't have a weakness to Ice/Electric/Ghost and have a really strong attack) can often come in while Trick Room is up take a neutral attack and KO with a STAB move but they tend to be 2HKO'd so they can't switch in.
If you don't have any of these just hit it as hard as you can as it sets up, and try to use some strong priority moves on it or stall out Trick Room by smart switching.