What should I do with this Omastar ?

Don't power it up. It is perfect as is.

Omaster is amazing!! Amazing, but it's strength is in boosting up gyms not in taking them down. And it will never be a good defender.

Your 1,250 CP Omaster can easily take fire creatures that are up to 3,000 CP. You may run into situations where there are 2 fire creatures at the bottom of a friendly gym and you can take them both with the Omaster getting a ton of prestige points.

Omaster when fully powered up doesn't get huge. It will never be an amazing creature for taking gyms down or for defense.

I have several different Omaster in the 800 CP to 1,600 CP range and they are by far my most used creatures for boosting up gyms.

My 800 CP Omaster can easily take down a 2,200+ Arcanine for 1,000 prestige per victory.

I would definitely not power that up. IMO the only pokemon worth powering up are the ones with a high IV, high CP and with a good gym defense moveset.

Stardust is so valuable and as you get bigger it gets used up so fast. Taking a mon from level 20 to level 40 is roughly 200K stardust so a player can only plan to take so many pokemon to max CP.

The mon's I power up are only to place in gyms that I think will stay long enuff to recover any stardust plus gold coin put in and then sum.

Im currently level 30 and collect on 10 gyms every day and I use the GC from daily gym collections for incubators and purchase lucky eggs periodically with real life money.

I have an Omastar CP 1486 with WaterGun + Ancient Power. Don't know what to do with him either.

I have so many water-type mons to crush fire-type mons but people don't put fire-type mons in gyms much :D

Btw, I find Seadra quite efficient as they are really easy to dodge, maybe a bit quickier than a Vaporeon.