Dialga and Palkia are the main stars of Brilliant Diamond and Shining Pearl, but as per tradition, it’ll be a long time before you can actually catch and use them. More specifically, you’ll encounter them after defeating Team Galactic atop the Spear Pillar.
How to Skip Dialga/Palkia
For most trainers, it doesn’t matter that you’re forced to fight (and catch) Dialga/Palkia now. However, if you’re a shiny hunter, you may wish to delay catching them until later. For example, so you can experience the rest of the game, without being stuck at the Spear Pillar for thousands of resets…
There are 3 ways (one is temporary) to avoid catching Dialga/Palkia:
KO Dialga/Palkia. After you defeat the Champion, Dialga/Palkia will respawn back at the Spear Pillar.
Run from Dialga/Palkia. This works like above, but is far more anti-climatic….
Let Dialga/Palkia KO your party. While this might hurt your pride, Dialga/Palkia will remain at the Spear Pillar should you return. However, you cannot progress the main story until you capture, KO or run from them.
Anyway, if you plan to catch Dialga/Palkia right here, right now, be sure to properly prepare. Check that your party is fighting fit and well-suited to take on the fearsome Legendary. Remember that you can access your Pokémon Boxes, if you need to swap in some different Pokémon. But you’ll be unable to restock on Poké Balls unless your whole party is KO’d.
Dialga (Brilliant Diamond)
If playing Brilliant Diamond, you’ll face off against Dialga, the Temporal Pokémon. It can use Roar of Time (a hard-hitting Dragon move that requires recharging), Flash Cannon (for Steel-type STAB), Ancient Power (weak, but has small chance of boosting all its stats) and Slash (not too scary, but has high critical hit ratio).
Defensively, you’ll want Pokémon that won’t crumple against Roar of Time and Flash Cannon. Sadly, most Fairies are out of the picture since they’re weak to Flash Cannon. Instead, your best bet is most Steel-types, although Steelix is risky since it has low Special Defense. Magnezone is a solid choice since it has access to Thunder Wave.
To weaken down Dialga, you’ll likely have to rely on not-very-effective moves. It may take a while, but Magnezone can easily handle this task, in addition to its Thunder Wave and walling job. Towards the end, you can swap over to a Pokémon with False Swipe, then swap back to Magnezone to continue walling. Or you could use Soak near the start to change Dialga to a Water-type so it doesn’t resist False Swipe.
Once Dialga is in the red, with a status ailment (paralysis is nice and easy), it’s time to start unloading your best Poké Balls. If you’re feeling lucky, you could throw a Quick Ball on turn 1 to see what happens. Obviously, the Master Ball is a valid option too, if you’ve got nothing else to use it on. Besides those, stick with Ultra Balls for consistency, Timer Balls after 10 turns or Dusk Balls during Night.
Palkia (Shining Pearl)
Meanwhile, Shining Pearl players will confront Palkia, the Spatial Pokémon. Similar to its counterpart, it can use Ancient Power (weak, but slim chance of boosting all its stats) and Slash (fairly weak, but has high critical hit ratio). Additionally, it’s carrying Aqua Ring (slowly restores its HP; annoying!) and Spatial Rend (a Dragon-type move with high critical hit ratio).
Palkia has no Water-type attacks, so your main Pokémon only really needs to resist Spatial Rend. Fairy-types will take no damage or you could field a Steel-type. Resisting Palkia isn’t too tough, it’s dealing with Aqua Ring that’ll cause headaches. You can prevent Aqua Ring by using Taunt (eg. the Chimchar line, Houndoom, Murkrow line and Glameow line). However, that’s only a temporary measure.
The other option is to use Will-O-Wisp to apply a Burn, which will cancel the healing from Aqua Ring. This is easier in the postgame, when you’ve got access to the Battle Park and Pokémon like Vulpix and Duskull. However, it’s obviously not feasible during your first encounter. Otherwise, you must grind in the Grand Underground for small spheres, then hope a Small Sphere trader is carrying TM61 Will-O-Wisp.
Realistically, you’ll probably just have to live with Aqua Ring. We suggest applying paralysis (with Thunder Wave etc.) near the start, to occasionally stop Palkia from attacking. After that, cross your fingers it doesn’t use Aqua Ring or prevent it with Taunt. If Palkia does use Aqua Ring, keep using False Swipe every so often to bring it back down to low HP.
When the conditions look good (Palkia has red HP and is inflicted with paralysis or maybe sleep), begin throwing your favorite Poké Balls. If you’re lucky, you could even capture Palkia on turn 1 with a Quick Ball. Or you could go the lazy route and use the Master Ball from Cyrus. If not, rely on Ultra Balls, Timer Balls after 10 turns or Net Balls since Palkia is a Water-type.
Having captured Dialga/Palkia, there’s one more thing you can do here. Head straight up to the back of the Spear Pillar. Past the steps, you can obtain an Adamant Orb for Dialga or a Lustrous Orb for Palkia. Give these to your newly-captured Legendary to hold onto and they’ll boost the power of their STAB moves.