did you check my calc?
252 Atk Orichalcum Pulse Koraidon Collision Course vs. 0 HP / 64 Def Tera Stellar Terapagos-Stellar: 390-458 (84.5 - 99.3%) -- guaranteed 2HKO
also terapagos can OHKO koraidon at +2 (which is surprisingly easy to get to)
+2 252+ SpA Tera Stellar Terapagos-Stellar Tera Starstorm vs. 0 HP / 0 SpD Koraidon: 344-406 (100.8 - 119%) -- guaranteed OHKO
This doesn't even begin to address the point I made on why Terapagos is ass. Jesus
First of all, calcs is a really bad point of reference because if that's your main basis, we might as well put Haxorus in S+ rank because 252+ Atk Mold Breaker Haxorus Scale Shot (4 hits) vs. 0 HP / 4 Def Koraidon: 344-416 (100.8 - 121.9%) -- guaranteed OHKO
Second of all, my entire point isn't about setup or calcs. It's the fact that you have to dedicate your tera slot which limits your flexibility. Tera is so valuable right not because it can make or break a game. One tera fairy might save your team from being pried apart from Koraidon while that same Koraidon can use tera fire to avoid burns and resist fairy or tera ghost to block extreme speed
And third of all, your entire calc argument is heavily flawed because Koraidon's literally doing over 80% with a move the it doesn't even use. More often than not, it uses low kick and those who might be feeling a bit frisky, close combat
Koraidon can simply U-turn out too since Terapagos ability works only once per game.
Not that Koraidon needs to switch out though
252 Atk Koraidon Low Kick (80 BP) vs. 252 HP / 0 Def Tera Stellar Terapagos-Stellar: 248-294 (47.3 - 56.1%) -- 82.4% chance to 2HKO
And this is just Low Kick. Collision course practically oneshots if it's chipped.
252 Atk Koraidon Collision Course vs. 252 HP / 0 Def Tera Stellar Terapagos-Stellar: 414-488 (79 - 93.1%) -- guaranteed 2HKO
If it's an offensive Terapagos set not running full HP...yeah things not looking too good for the turtle
Btw, Scarf Koraidon outspeeds Terapagos even after Rock Polish. Since Terapagos lacks recovery outside Rest, it's not that hard to overwhelm
Can someone explain to me please why a giant turtle that's sitting on a metal umbrella with giant gems around it somehow doesn't take full damage from low kick?
Putting more EV in defense doesn't change anything, it still gets 2hko by Koraidon no matter what. It's very weak as a setup sweeper, and there are over a dozen better setup sweeper in the tier. It's bulk isn't anything special, since it has no resistances or recovery, unlike actual tanks like Necrozma Dusk Mane/Ho-Oh.
You are severely overestimating its usefulness, even with tera it struggles to compete with Ubers Mon without tera, and this thing practically needs tera to not become completely deadweight (its base form would be average in OU, if lucky).
Since you are not very experience, I suggest you reach high ladder and become more knowledgeable on Ubers meta. A/A+ is reserved for the top Ubers mons, Terapagos will be lucky to go past C+ (I see it becoming unranked eventually, since nobody likes a tera hog which is a shitmon otherwise).
Also, you don't even need to reach high ladder. Even just being 1500 elo consistently will be enough. Uber ladder is broken because a 1400 players gets matched up against 1600 to 1800 player. Not all the time but it isn't uncommon either. That's why I think 1500 consistently would be a fine choice because that's the rating where people build teams to handle the meta but often times has trouble executing their team's gameplan