Gosh this thread is silly. Like someone already suggested, you just equip your TH gear whenever you use box step. Box step doesn't even have to land. The fact that you used an ability on the mob with your TH gear will put TH6 on the mob.
And wtf? Seriously why is there still even discussion about Thief's Knife in Dynamis? If you are still stupid enough to use that knife in Dynamis then you are beyond help and you need to stop trolling these forums.
You don't even need spellcast. You can make a macro:
input /equip hands "Assassins' Armlets +2";
input /equip feet "Raider's Poulaines +2";
wait .2;
input /ja "Box Step" <t>;
wait 1;
input /equip hands "Good TP hands";
input /equip feet "Good TP feet";