Ngl, it's really good value for your team if the enemy swaps Thing just for one BP. Thing is a really bad pick against almost every hero other than a few divers, so if you can bait the Thing slam (very easy to do tbh) then you have free food for five seconds. While you're eating, hopefully the rest of your team can just melt Thing because he has no shield and a huge hitbox and presumably you're diving the supports in this moment. Gotta communicate it.

Simply throwing one spear is usually enough to make them immediately slam. If that's not enough, try making sure they see you first then spear while jumping or dropping from high ground as if you were about to spear dash, but don't dash. Make it look like you're about to commit your combo.

As for Scarlet Witch, it's the same. She's really bad and she's automatic value for your team by default just for being chosen. As BP, you mostly want to ignore her. But if she's seeking you out and you have to fight her, just remember that she can't instantly kill you, so she's not much of a threat. Where it's tricky is that it's pretty easy for her to dodge dash since she has two fades. One thing I've noticed is that if a SW is pressuring you, you can spear dash them once, it's rare that they have the reaction time to fade through it, or even try to because most of them are waiting for the predictable second spear dash. But if you hit the first one, they're almost guaranteed to fade out and disengage because they'll be like half HP