How to unlock Groudon
Getting Groudon means pulling the Hoenn banner and hitting its 5★ slot. The 5★ band takes 1.5% of all pulls and holds 103 partners, so one specific Groudon works out to about 0.0146% per draw. The 70-pull pity counter guarantees a 5★, but it does not target Groudon specifically, so a guarantee can still resolve to another 5★. A duplicate Groudon refunds 300 coins at the 5★ rate.
Groudon stats and rank
Groudon carries 115 HP, 116 speed, a ×1.52 attack multiplier, ×1.52 physical, ×1.25 special, 140 defense and 90 special defense, 670 in total. On the three-stat percentile this is 87.0 out of 100, which puts Groudon at rank 30 of 966 and in tier S. What separates it from its neighbours is shape, not size: its strongest slot is physical multiplier (99th percentile) and its weakest is speed (75th percentile), so it spreads its value fairly evenly.
Best moves in Groudon's pool
At move level 5 the pool is led by Body Slam at 75.6 damage per 0.71s (161.3 effective DPS, ×1.52 multiplier), then Tackle at 33.6 damage per 0.38s (136.2 effective DPS, ×1.52 multiplier), then Quick Attack at 44.8 damage per 0.56s (121.1 effective DPS, ×1.52 multiplier). 10 of its 13 damaging moves scale with the physical multiplier and 3 with the special one, alongside 4 support moves. Its physical multiplier is the higher of the two (×1.52 against ×1.25), so physical moves convert better. The top option beats the second by 25.1 DPS, which decides slot one and leaves slot two as a judgement call.
Evolution cards for Groudon
Its starting move, Mud Shot, evolves into Earth Power once the move is at level 5 and you hold the Power card, which boosts attack for that run. A level 5 move without Power in the run is still just a level 5 move. 13 of its 17 moves carry an evolution card, across 5 distinct cards (Vigor, Sprint, Power, Reflexes and Armor). The strongest of them is Body Slam at 161.3 effective DPS, so that is the one worth holding a card for first.
Groudon compared to similar partners
The two closest stat lines in the roster are Zekrom (+0 HP, +0 speed, same attack multiplier, +10 total) and Solgaleo (+17 HP, +3 speed, -0.07 attack, +10 total). Against Zekrom, the two are level on speed and the two carry an identical ×1.52 attack multiplier, and the rank gap is 3 places. The exchange rate is the point: nothing here improves on both lines at once, so neither side is a strict upgrade and the move pools have to break the tie.