Turtle Advice

[Demon]

Well I don't play Scally that much, but . . .
I'd say hit it immediately. When the shell first sets up is when you have the most troops capable of attacking the position. Afterwards, your troops are gonna get forced out and the enemy will be able to easily reinforce the position. So early is your best bet, especially if their defenses are hastily constructed and/or incomplete.
This means vision is important (vision is always important, but especially so here). Placing a runner directly behind your soldier wall increases your vision one hex, showing the range where a shell could be placed to hit your wall. Also don't be afraid to sacrifice a runner to peek behind enemy lines (if you can). There's a reason they're so cheap; they're meant to be expendable!

So kinda what Uber said, I guess . . .
Unfortunately, if the shell is placed properly, it's going to be a good trick even touching it, let alone killing it.