I'd say Thunder is still a very straightforward "kill it before it kills you" element. Technically you can play a more "control" build now that Dian is out, I guess. Or a Phantom Elemental Lord Thor build, I mean, that's always an option.
Either way, the problem with Thunder is that unless you have Thunder Aphro, Fire just does the whole "kill it before it kills you" thing way better. With Kirin, if you want to be serious with Thunder you need to be looking at the first MT of 2020, which will hopefully give us mere mortals Thunder Aphro. So, if you want to commit to that build, grabbing Marduk/Dian/Raiko would probably be a good option for now.
EDIT: Oh hey, I just noticed that the guy we're talking about has Kirin, Thunder Aphro, Marduk and Raiko, so is only missing Dian.
If his Thunder feels squishy after that, he'll just need more Assault/Defenders in his Grid (or a Phantom Gun Grid, I guess?).