Best stand-off weapon for...

[Imp]

Err nope think WW1 heavy continous bombardment or storm the trenches preferably both is about it, some better arty but still need to plan & keep hiting, so concentrate on one area at a time rather than spreading the love.
On the arty going all big guns is in my view not the best way, they kill IF they hit but smaller stuff suppresses more & can risk keeping it going longer when your troops are near. So hit with a mix of to start & once ready to assault move heavys to next target