"Safe" is a relative term here. You need to have good/precise spacing to effectively wall... but really smash 4 isn't a game where walling is very effective. that said - here's a short list of good spacing, walling tools.
-SH Nair: Comes out on frame 5, this is your fastest aerial. use this to pressure, use it as a landing option to stuff grabs. the backwards hit of this can knock at a good angle for gimping.
-Landing / SH Bair: Excellent range and power - use this to pressure shields and stuff approaches. the range on this also makes this very useful for follow ups off of AC dair or down throw (at low to mid percents)
-Auto Cancelled Dair: Incredibly good spacing tool and can invalidate certain approaches like dash grab or SH aerials from characters with less range. This tool is especially useful in stuffing grab game of literally half the roster. Like seriously, you need to abuse this tool against characters with really good grab games (luigi, mario, falco, fox, sheik, zuit) to stop them constantly throwing projectiles at you and baiting out shield *shudders with anger*
-Cross Slash: Really good against SH positioning pressure - extremely good mixup tool because it deals 19%
my advice here is not to think of it as walling. this is not melee marth. think of it as spacing.... (maybe i just like that word more lol)
