If it were mine I'd remove some of the paint from the dented area and fill in the dent with braze and then repaint as little as I could get away with.
Obviously the heat involved in this is gonna cause some paint damage but it will ultimately save the frame. If you find a local welding company they should be able to flow some braze in pretty neatly - it'll be up to you though to sort the paint out afterwards.
The dent is too big to be filling with a body filler and if you braze it your not going to stand the risk of the filler cracking if the frame is used a bit.
the other option is to put a sticker over it and just ride it
1988 Emmelle Leopard
1991 Marin Muirwoods
1992 M-Trax 1500
1992 Giant Cadex CFM2
1992 Saracen Rufftrax
1996 M-Trax 450Elite Titanium