Glyphoshate to kill it, then see if there is anything left in the seed bank to shoot up. Once dead maybe disc it to help expose more soil? If I remember correctly from trying to put together a woodcock project in an area reed canary grass took over, best to try to kill it late fall when its getting ready to go dormant, but check on that. If no seeds left in the soil bank, you'll have to do some planting.
I asked a similar question to the experts at Ernst Seed. Here are their replies.
"If cattails are too thick, it will be hard to get anything in. An herbicide called Rodeo (aquatic safe form of Round-Up) might be used to control cattails. After a kill, and if the site is chronically saturate but not inundated, the site could be overseeded with our ERNMX-122 FACW Meadow mix at 0.5 lbs/1,000 sq ft (20 lbs/acre). A cover crop should be applied with it. If planting 1 Sept to 30 May use 0.7 lbs/1,000 sq ft (30 lbs/acre) of cereal (grain rye). Otherwise use 0.25 lbs/1,000 sq ft (10 lbs/acre) Japanese millet."
"The Wildlife Food and Shelter Mix is not a bad mix either. The FACW Meadow Mix is an herbaceous mix whereas this is more of a shrubby mix with some herbaceous species.
I would spray the cattails when there is about 6 inches of new growth in the spring. Wait a week, then hit any misses. After that, mix the seed with a clay based kitty litter (1 volume seed per 2 volumes kitty litter) and broadcast on areas where there is exposed mud. Most of our wetland seeds will float if broadcast on water."
"I have not had a chance to see performance of any of our shrubby mixes. My gut feeling is that within 5 years I would see some woody growth to five or six feet.
Seeds of most of the shrubs want to pass through a winter before they will germinate. Following germination, growth rate will be greatly affected by temperatures. A growing season that is 65-85 should give greater growth than one where daily highs are in high 90’s."