The Lord Ruler can still try and crush the thunderclast with his weight, and I think it's fine to assume he brought of source of metal to manipulate like cins if he wants to push or pull.
We have no idea if Ishar can counter Atium (If you have a WoB, please share) and that still doesn't change that he can kill Ishar in a split second. As for soulcasting, I have a made a thread where it's clear that TLR bands would be filled to the brim and very hard to soulcast in any fastidious matter. As for TLR himself, BS has said it's hard to soulcast people because they are more invested, and TLR is a sliver, so he is VERY invested to the point of resisting the pull of the beyond for a very long time, so I don't know how much of an option that would be for them.
@seriodor Okay, fullborn and mistborn aren't the same thing, please clarify which you're talking about, because Fullborn would be a storming Lord Ruler, which as have discussed would wreck radiant and possibly even a herald.