This team was built around Mega Gardevoir + Weavile, the latter of which trapping Jirachi, the biggest obstacle for Mega Gardevoir just obliterating everything slower than it, while also helping to chip Blissey with Pursuit to put it in range of two Psyshocks or a Focus Blast + Psyshock as it comes in. Focus Blast was chosen as Weavile is good enough to put Blissey in Psyshock range. From there Mega Gardevoir destroys many stall teams on it's own, even without a utillity move in the 4th slot. Focus also hits rising defensive checks, such as Mega Steelix and Empoleon, the former of which being problematic for Weavile. Weavile's Knock can also take away the scarf of any Urshifus bold enough to switch in on it, making it a complete non issue for this team, and is one of the toughest moves in the meta to switch into right now.
Amoonguss was added as it is the best answer to Urshifu in the tier (the scarf set in particular, which wrecks the core and honestly most of the rest of this team RIP), punishing its Surging Strikes with Rocky Helmet, taking next to nothing from everything bar banded Zen Headbutt (which you live anyways), and healing off chip with Regenerator. Notably, Regenerator completely negates any chip U-turn from Urshifu accumulates. It also blocks Celesteela's Leech Seed, hindering it's recovery greatly and making it much easier to wear down for Mega Gardevoir's Mystical Fire. I rarely found myself clicking Synthesis so I ditched it in favour of Giga Drain to better deal with Mega Gyarados between it and Clear Smog.
And now, for the subject of this cycle, Infernape. SD and NP are terrifying breakers, sure, but people often forget that Infernape has a sizeable amount of defensive utillity with defensive sets, being able to set rocks while checking Weavile, Bisharp, and Scizor among other things, and more consistently than Cobalion due to having recovery in the form of Slack Off. The set outspeeds Heattom, set rocks vs it, then U-turn as it Defogs into Mega Gardevoir, allowing it to chunk something. Fire Punch is chosen to reliably OHKO Weavile if something else knocks off Infernape's helmet, which is important because Weavile blows chunks in the team otherwise. Rocky Helmet allows it to freely switch in on Celesteela's Heavy Slam and Jirachi's Iron Head and chip them down for Mega Gardevoir. Infernape also checks the two with Fire Punch and can set rocks as they switch out, further chipping away at the two.
Blissey pairs exceptionally well with Amoonguss, dealing with most of the special attackers that break Amoonguss and having many of it's breakers checked by Amoonguss, forming a core that checks a huge portion of the meta. Blissey on its own is typically not sufficient Mega Gardevoir counterplay for the long term, but in this case it can Teleport into Weavile, which can trap MGarde, OHKOing with Pursuit if it switches out. Finally, Rotom-Heat is the team's defogger, ground immune, and secondary Scizor check. Weavile's Ice Shard is enough to check Salamence so there is no need to run Thunder Wave or Will-o-Wisp, and Mega Gardevoir + Weavile is all the wallbreaking you will ever need so Nasty Plot isn't that necessary, and as a result Pain Split was the best option, allowing it to check mons more consistently and significantly weaken any Blissey that try to come in on it to help Mega Gardevoir.
The team can struggle vs opposing boosting Infernape but the team's own Infernape can sort of check it with Helmet chip and bulk, spamming Slack off to put it in range of Weavile's Icicle Crash. That being said SD Mach kinda cleans house vs this team so make absolutely sure you don't let it set up for free. If need be you can suicide with Amoonguss to obliterate it's health with a combination of Rocky Helmet + Flare Blitz recoil, but I don't recommend it if their Urshifu is still alive. Fortunately, many boosting Infernape are ditching Mach for U-turn or Gunk Shot due to Latias and Mega Altaria, so it isn't necessarily GG if it shows up. Aside from that, this team is pretty solid from my experience, and Infernape is a bad matchup for Balance teams to begin with.