Lots of thoughts running through my mind. First off, are you from the bank or boat? If you are stranded to the bank then you have no choice. I'd find shoreline cover, be stealthy both walking and with your presentation. Big girls should be shallow right now anyway.
If you have a boat, I would fish this lake/pond year round and at all different depths depending on water temp/time of year. If there was 1 giant, there's others, and during the summer, I would bet most would go deep. I also think the clarity may get better the deeper you go.
As for now, I too would be shallow with a chatterbait, big squarebill (BIG, like the KVD 8.0), big swimbait or a jig.
And no one has asked, but we are all thinking it; was the fish weighed on a scale or was the 18 an estimate?