I use it to throw off my opponents a lot of times because no ever expects fludd in the middle of a fight. I sometimes also only charge it halfway at the beginning then start charging again as I descend to the stage later on, baiting them but you can flick left or right to roll then punish or shield or shield>grab if you're facing them. Also shield>OoS up smash/up B/bair or just shield>jump away. Quite useful. They're a lot more likely to run in if you're not facing them.
In non-edge guarding situations it takes about a half charge to make a significant impact on the opponent. If you knocked them into the air or they are coming at you from the air in a way they think you can't cover, just aim and blast them with the fludd (you can always charge another), it will stop their momentum in air and if they're close enough you can usually get an Fsmash or other punish.
Also if I'm getting frustrated with an excessive roller, a projectile ledge camper, or a fast character with good spacing and baits, I'll just blast them sideways. It really helps if they were already moving in that direction. Even if it seems like a waste of a fludd or a random move the opponent is now somewhere they did not want to be and you put them there, either way it's a quick way to reshuffle the current situation.
Since Mario can get juggled bad sometimes a half charge or more fludd and DIing backward makes a good escape, especially if you've already used your second jump and don't just want to Up B and hope for the best.
For character specific things, fludd (ANY charge) repels all of Pac-Man's fruits except the key, Link/Tink's bombs, but not Peach's turnips no matter how much it seems like it should.