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Honestly Weavile and Garchomp don't seem too threatening with Rillaboom, Heatran, Gastrodon, and Taunt Tornadus, Perhaps different playing will help. Try to preserve Heatran's health for Weavile, and use Rillaboom to counter Garchomp.
I feel that weavile is a problem mainly because if flame body doesn't activate, then weavile will eventually overwhelm heatran. To be fair, this has only happened a few times, but I still feel that it is a concern.
You're right about garchomp though. I've been playing quite conservatively with rillaboom, which is probably why garchomp was problematic.
Correct, if your opponent gets lucky using Weavile or has something that forces your Heatran to take damage, you're in trouble. To remedy this, I'd try to ask a big question: What is your team trying to do? At the moment, it seems like a semistall, where its goal is to chip and sit on the opponent's team, waiting for opportunities to get in your breaker (Blacephalon).
If your goal is to break, you may want to consider something over Blacephalon. The best semistalls all have a pokemon that always or almost always breaks, it just needs time (which the semistall provides). Some notable examples are NP Glowking , and Calm Mind Tapu Lele. Blacephalon is not on this list. Even with Specs, its hardwalled by Ttar and has to Trick Choice Specs so it cant break as well.
Currently your team cannot break many teams. So I'd lean one side or the other. Try to wall better, or try to break better. Alternatively, you can restructure your team to have a more solid breaker than Blacephalon. Perhaps Colbur Berry Nasty Plot Slowking-Galar would help with Weavile and be a more consistent breaker than Blacephalon.
I've changed blacephelon to glowking and made some other changes, but the team hasn't really worked out. I feel it's worse than the original blacephelon team. Glowking hasn't done anything in any of the games I've played except for use trick and sometimes KO one mon.