A sub-seeder sets up Subs. No matter how much power there is in an attack, it means little if it only hits a Sub whilst his Health gets drained away. Subseed Skymin wouldn't swap in on something so powerful, taking SR damage. It would come in, after one of its teammates gets KOed, on a pokemon that poses little threat to Skymin and should swap out to a counter. Skymin would use Leech Seed, hitting the switch-in (I will refer to this pokemon as the switch-in for the rest of the post). She will gain Leftovers recovery plus Leech Seed recovery. Skymin would be forced out, usually facing a OHKO threat, but she gains Health from Leftovers and Leech Seed that heals most of the SR damage she received, whilst the opponent receives more damage than Skymin thanks to Leech Seed and SR in that 1 turn, even if he swaps out immediately.
If the switch-in has Roar or Whirlwind, the above still applies. The only difference is Skymin may stay in to take the Roar, meaning the pHazer takes another round of SR damage.
If there is no switch-in (that is, the pokemon that poses little threat to Skymin stays in), he should get beaten by the Skymin until he is eventually forced to swap out, and Skymin gets to Leech Seed the next pokemon, usually with a Sub up.
The main exception in which Skymin takes less damage than the switch-in is if the switch-in has Leftovers. The common switch-ins (in term of usages - that is Zapdos is #4 in October Usages, whilst Registeel is far from OU) that might hold Leftovers are:
- Scizor (who generally prefers LO - Leftovers Scizor cannot 2HKO Skymin with Bullet Punch, and whilst Skymin will 3HKO with Air Slash, Leech Seed Recovery put the odds in Skymin's favour)
- Zapdos (takes just as much damage as Skymin from SR, so it does take more damage than Skymin)
- Blissey (who gives an absurd amount of Health from Leech Seed)
- Tyranitar - will get into KO range of Seed Flare before he has a chance to beat Skymin, and he cannot outspeed Skymin without Rock Polish. If Ttar does use that move, Skymin will probably switch out after her Sub gets destroyed, if she doesn't, Tyranitar has died from Seed Flare.
- Metagross - yes, it takes less damage with Leftovers
- Skarmory - yes, he takes less damage with Leftovers
- Tentacruel (Liquid Ooze variant definitely has the upper hand. Clear Body may not. The standard Health for Tentacruel is 352, whilst Subseed Skymin has 341. Skymin loses 85 per turn from Substitute, while Tentacruel loses 44 Hlth per turn from Leech Seed to Skymin. Factoring Leftovers, Skymin loses 20 Hlth per turn, whilst Tentacruel loses 22 Hlth per turn. Eventually, Tentacruel is in KO range of Seed Flare before Skymin faces a KO threat.)
SS is the other exception, assuming that the switch-in is immune to SS.
Any Subseeder can do all the above, but Skymin has an easier time at it thanks to threatening more pokemon that other Subseeder users due to Skymin's high SpcAtt (2nd only to Roserade) and high Speed (top), STAB Air Slash and Seed Flare's raw power. Taking hits in these scenarios is irrelevant, so Skymin > Celebi and Landmin.
I also want to be notified of this, as I am away from Wednesday to Saturday too.