Yeah I think Bennet just sucks at doing that, going to level up the free pyro spear user I think.
I use Xiangling and her bear is really unreliable for shields since mages can teleport and the bear just breathes fire in front of it. The whirlwind works, but is an ult and as such energy consuming (and doesn't last that long unless you have the constellation that extends it with 40%).
For destroying shields with fire I'd recommend Amber. Destroying shields does work great with low level characters, and while you don't need to level them up it can be nice to max a flower artefact on them and then just put on a few low level %hp artefacts. Don't need to worry about the sets, just a leveled flower + level 1 to 5 artefacts with HP% where you can. It will boost even level 1 characters to 10kish hp so they are less likely to get oneshot.
Also, mages with water shields can be frozen even if they are inside their shield.
HIGHLY recommend using any and all primo gems you have to get Ganyu for this banner. She does so much more damage than any other character that she feels broken.
I now have her at level 90, with a level 90 amos bow and maxed her regular shot talent to level 10 (since it also boosts charged shots). Equipped her with a water set (since the 4 piece set adds a constant damage increase to charge shots and isn't situational like wanderers / cryo). She crits 50+% of her charged shots.
With no elemental reactions she does around 40k per charged shot with a crit. If they're on fire she does 80+k ish. Unless an enemy has cryo resists I don't even swap her out. She kills all content with a few arrows, hillichurl camps she can clear in one if all shards hit. You can do the super fancy 100+k crit with Mona that requires juggling a lot of characters with charged ultimates, or I can shoot the enemy 3 times with Ganyu for the same damage taking less time than it takes to swap between characters to do the Mona thing. Add in the ultimate that drops 5k-8k shards for 15 seconds....