Deathstroke I can't comment on yet, I've played several decent DS players in tourney/casuals but I don't own the game yet to go into the lab and test specific punishes/ranges. The matchup SEEMS pretty even but it definitely requires some precision on Aquaman's part.
As for Grundy...in my experience, I think it's generally better not to attack a GOOD Grundy at all if you can help it. If you get a knockdown from a combo that leaves you close, and have a nice chunk of life to cover your ass on a wrong guess, feel free to test him a bit with b+1 or b+2 to see what his blocking tendencies are (just make sure you can FINISH any combos you start, esp. off b+12, you don't want to leave Grundy standing that close if you can help it). But if you're close to even on life, don't take risks. The longer Grundy stays standing while you attack him, the longer he has to figure out your attack patterns and either armor grab (if you stay on the ground) or anti-air (if you start jumping in). Just try to space yourself for d+2 anti-air and keep retreating carefully.
If you've kicked him out to safe splash(DB2)/trident (BF2) range, DON'T be a hero. Just be lame, stay on the outside, and space yourself for anti-air (if Grundy jumps) or reaction jump (if Grundy goes for a grab) when he gets past splash/trident range. The only thing you have to watch for at splash/trident range is swamp hands, and you can block that pretty easy on reaction as long as you don't get too reckless with splash/trident.
