The only way to get a chancey or a blissey with Seismic toss is if you do this:
-Get a shadow Chansey from Pokemon XD: Gale of Darkness
-Tranfer the Chancey to Emerald ONLY
-Go to the Battle Frontier and go to the tutors
-For 48 BP, get Seismic Toss taught to Chansey
-Put it in pokemon storage system
-Migrate from Emerald
If you don't want to do all that, then get game shark.