I was thinking about building a team with SubBP Celebi for a long time. Even before the removal of the BP ban I thought about using U-Turn Celebi in a similar structure to what I ended up with. I think Celebi had been underrated for some time, with people putting it in Tierlists as low as D-Tier, which I strongly disagree with. While it may be not slamable on any team, it can use a variety of sets and shines through its typing and synergies. Very good players like Fakes and Void used teams with Celebi to high success. After the lift of the BP ban, I was a bit disappointed that no one would try something with it, I think I have only seen two matches (one in DPPPL and one recent match of Pideous), where SubBP Celebi was used. I discussed this specific team with well known players but did not receive a lot of feedback, so maybe it is best suited for this contest.
Building process:
I was inspired by the well known and very strong Gen5 OU Team (Landorus, Terrakion, Tyranitar, Celebi, Keldeo, Excadrill), which has the idea of using BP Celebi as a pivot to position its strong and fast sweepers. It follows the philosophy of using a bulkier team which utilises combinations and synergies to break stall instead relying on a single mon for that. Of course I had to make big adjustments as the concept seems to be better in Gen5 because of the new mons, which are not available in Gen4. Funnily enough the team then does not even use a single Gen4 mon but instead consists of a nice mix of two mons from each of the first three generations. The building process was as follows. For me it was immediately clear that I wanted a trapping core of Magneton/Magnezone and Tyranitar to remove Skarmory and pressure defensive teams. I decided to settle for Scarf Magneton, which also acts as a revenger for the team. I like Mixtar here for Pursuit and also because sometimes you create situations versus stuff like CB Sciz where they have to decide between using BP and getting trapped or U-Turn and getting lured by Fire Blast (see Replay 2). Furthermore it can take a hit or two versus most special sweepers. For Celebi, I decided to use Grass Knot as the attacking move, which threatens Swampert and Hippo. Together with the trapping core, pretty much all common PHazers are covered. Next I added Flygon, as I wanted to have something to receive BP which could pressure Jirachi. Originally I used CB (as you can also see in the replays below, which are a bit older), but I decided to change it up to the less common physically based set with Toxic, which may give me the upper hand in the long run versus Hippo, Glis, Def Zapdos and so on. With Skarmory removed, only Bronzong resists all of Flygons moves, which however is most of the time setup fodder for Celebi again. I decided to add my own Jirachi to set up SR and for its resistances and DD-Gyarados because of its typing and as a further wincon.
Replays:
The replays below are from ladder versus good opponents and show the strengths of the team very well. In the first match, the combination of trapping Skarmory and repeat Celebi passes creates too much pressure for the opponents stall team. Celebi + Trapping punishes passive teams which use e.g. Resttalk Rotom, Bronzong, Clef (note that you create 101 HP Subs which cannot be broken by a single Seismic Toss). Also Celebi can be very nice in gluing the team together, similar offensive teams oftentimes have trouble with Swamp, Gyara etc. but here these get handled easier. The second replay shows the trapping core in action, which gives me a quick 6-3 advantages, from which it would have been very hard to come back.
https://replay.pokemonshowdown.com/gen4ou-2016261934-svhm059p3t2rdv3bcop0x9a7mu2xjt9pw?p2
https://replay.pokemonshowdown.com/gen4ou-2016158144
Weaknesses:
Shed Shell Skarmory is of course a big problem, as it denies passes, walls Flygon and does well versus most of the team, but that just lies in the nature of the building process.
Roserade cannot be trapped and sets up Spikes and TSpikes, which can both be very dangerous.
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Depending on the used Ttar berry either Gengar (which can also deny Celebi passes once all 4 moves are known) or strong water types like Cune/Empo/Starmie are difficult to deal with, personally I believe Passho is better here, so you have to play around Gengar a litte more.
Infernape and to a lesser extent Loom (depending on the sets they are carrying) can be dangerous.
Other options:
Psychic on Celebi (worse vs Hippo/Swampert which carry Roar sometimes, better versus Gengar, Nape, Loom, ...), different Ttar item (Chople...), Fire Punch on Jira may not be necessary, Flygon set could be changed, a Paraspam approach to the team may also be possible
Closing words:
I am quite happy how the team turned out. Overall it is slightly stronger versus more defensive and slower teams, but may occasionally have some trouble against more aggressive teams. If you like to take a bit of a risk in the builder, you may be rewarded by very favorable matchups. Thanks for reading!