For a 25' run, you are looking at about a 3" drop (1/8" per foot for 3" pipe).

You basically have two options. Cut a trench or build a floor on a floor. If you have the headroom the second floor, that is a good alternative. You could add some radiant heat to it if you want. Trenching isn't that bad. Just bite the bullet and rent a wet saw, make the cuts, then bust up the concrete and start digging.