Jellicent @ Colbur Berry
Ability: Water Absorb
EVs: 248 HP / 172 SpD / 88 Spe
Calm Nature
- Recover
- Will-O-Wisp
- Taunt
- Scald
Latias-Mega @ Latiasite
Ability: Levitate
EVs: 248 HP / 8 SpA / 252 Spe
Timid Nature
- Calm Mind
- Dragon Pulse
- Psyshock
- Roost
Jelli has been receiving more attention lately given its ability to wall Volcanion and other popular threats like Keld. While Mega Latias does perform well versus both of these Pokemon, it doesn't like being burned by Scald and Steam Eruption because being inflicted with said status cuts its longevity. As such, it's appreciative of Jelli's ability to serve as the main switch in to these attacks, allowing Mega Latias to be used more conservatively to avoid potential unwanted burns in several situations.
The above aside, these two work together in several other ways. For example, Mega Latias dislikes running into trappers like Tar and Weavile. Jelli has a tendency to invite these Pokemon in, and thanks to Colbur Berry, it's able to live their Dark STABs and retaliate back with a Will-O-Wisp. With Tar and Weavile burned, Mega Latias has an easier time sweeping with CM. Jelli also does a nice job dealing with other trappers standing in the way of a CM Mega Latias sweep, such as Mega Sciz. Being able to burn more defensively oriented obstacles to CM Mega Latias, like Ferro, is another plus. In exchange for this help, Mega Latias helps Jelli by handling several annoying Electric- and Grass-types. Examples include Breloom, Mega Venu, Mega Manec, and Thund-I. Being able to take care of other Pokemon capable of overpowering Jelli, such as Zard-Y, is also nice.
CM Mega Latias has been getting more love over BoltBeam sets and Jelli's viability has recently jumped, so the core represents some modern day trends. They work pretty nicely together and have some nice defensive synergy between them. Mega Latias and Jelli also fit nicely on the balanced teams they tend to end up on. They pair well with many other balanced staples like Ferro too, who appreciates Jelli to stop spinners like Starmie from removing the Spikes that do a nice job of wearing down Mega Latias' checks. Other classic balanced staples like Clefable, Heatran, and so on, also pair nicely here. This gives you a few options to choose from when building/picking partners to cover this core's weaknesses.