Terrakion in general is a jerk to your team. The SubSD set has a good shot at killing or heavily damaging something every time it gets a Sub up, the Rock Polish set can sweep late game with SR down once Garchomp drops to ~80% and Jirachi to ~70%, and the Choice Band set pretty much blows holes into everything just by spamming Close Combat (even Thundurus-T can be OHKOd after Stealth Rock).
Now a couple of changes that I think might be able to help. First, perhaps you could try an Adamant LO Mamoswine somewhere? I'm not sure where it would fit, but this will let you OHKO Landorus-I and Venusaur with SR once it's dropped to about 85%. You'll need to make sure to get SR up as quickly as possible, but this will also give you another answer to DD Dragonite. Not only that, but I personally think it fits your team quite well. After all, not much is stopping a Mamoswine that has been passed a Shell Smash, bar a couple of priority and Scarf users.
I also think that you could try an offensive Landorus-T. This would give you an excellent check to both Terrakion and SD Lucario, especially those without Ice Punch. You could run a EQ / SE / HP Ice / SR setup over Terrakion, which could take over that role as the SR setter and perform decently as a SS recipient. This would make you a bit more weak to Volcarona, but you still have Scarf Chomp to help with that. As a matter of fact, running Rock Slide > Stone Edge on Garchomp could help make it a bit more reliable as a Volcarona check. You do lose the ability to guarantee a OHKO on a bulky QD set at full health, but that shouldn't matter as long as you can keep SR down.