That's something I had considered (pretty sure Drizzt did something like that vs Icingdeath... wait he doesn't have lightning bolts! Can't remember how he did that), how did you rule the stalagmite, btw? Dex save vs half, did you really fluff the damage up or?Now that you asked I am realizing that everyone I play with either learned the game with my back in 1e or learned from me during 4e and 5e (and I treated those games pretty much the same as 1e). So that problem solving mindset was built into our RPG playing DNA. So I don't know how to make players thing that way.
Here is an example of what I am talking about though:
We had a group of 5 lvl 20 PCs go up against Tiamat. They had previously learned that she was impervious to all but the most powerful of spells (aka "Limited Spell Immunity" - unaffected by spells of 6th level or lower and dis/advantage on all others). Therefore they focused on defensive and buff magic. The wizard also used a lightning bolt to knock off a stalagmite above Tiamat to inflict damage. They didn't try to cast magic directly at her so much. They did try some illusion magic too, but didn't realize that she has truesight