Personally, I'd say go carefully! A tunic may emphasise the very bits you want to hide and just make you look "square".
Maybe you would be better off with a layering look- a long fitted T shirt, ( just reaching your hips) with a cropped or shorter cardi over, or even a T shirt/top with a "boyfriend" length cardi, which you can leave open, so it won't cling to your bum?
If you want to disguise your bum, then draw attention to your top half using cropped or tied cardis, beads, scarves etc, a good hair cut and a bit of slap, and wear dark fitted jeans.
Generally, I think that baggy or unfitted, or "disguising style" clothes on anyone who is trying to look slimmer, simply make them look bigger. Especially ankle length, gored skirts. Far better to go fitted, not too tight though, and make the best of your good bits!