This answer assumes that the harsh sunlight is gone or overwritten by Primoridial Sea. Which is another reason to not use Water Pulse because it's hard to get rid of harsh sunlight since everyone doesn't carry (primal) Kyogre. But honestly if you get Primordial Sea off then Water Pulse will basically be the same as STAB.
You are right, it does cover ground and fire but, at 60 base power without STAB then I don’t even know if it will be able to OHKO a (primal) Groudon because if it doesn’t then it will OHKO Lucario also, (mega) Lucario has higher attack than special attack. If you still want to use Water Pulse in your moveset the I guess that you gotta make your own moveset.
In conclusion, Earthquake is is a better cover for Lucario’s weaknesses or even Ice Punch for Ground.