Pokemon that have a dual typing which have 4x resistance to a type which is resisted by grass will effectively get a 8x resistance when afflicted with Forest's Curse. These typing are resisted by grass:
Electric, Grass, Ground, Water
NOTE: Dual types with Grass-typing won't count because Pokemon that have Grass-type already can't get afflicted by Forest's Curse.
Dual typing that have a 4x resistance to Electric, which effectively gets to 8x with Forest's Curse added:
/nds electric, dragon:
Ampharos-Mega, Dracozolt, Zekrom
Dual typing that have a 4x resistance to Grass, which effectively gets to 8x with Forest's Curse added:
/nds fire, poison:
Salandit, Salazzle
/nds fire, flying:
Charizard, Fletchinder, Ho-Oh, Moltres, Oricorio, Talonflame
/nds fire, bug:
Centiskorch, Larvesta, Sizzlipede, Volcarona
/nds fire, dragon:
Charizard-Mega-X, Reshiram, Turtonator
/nds fire, steel:
Heatran
/nds poison, flying:
Crobat, Golbat, Zubat
/nds poison, bug:
Ariados, Beedrill, Dustox, Kakuna, Scolipede, Skorupi, Spinarak, Venipede, Venomoth, Venonat, Weedle, Whirlipede
/nds poison, dragon:
Dragalge, Eternatus, Naganadel
/nds flying, bug:
Beautifly, Butterfree, Combee, Ledian, Ledyba, Masquerain, Mothim, Ninjask, Pinsir-Mega, Scyther, Vespiquen, Vivillon, Yanma, Yanmega
/nds flying, dragon:
Altaria, Dragonite, Noibat, Noivern, Rayquaza, Salamence
/nds flying, steel:
Celesteela, Corviknight, Skarmory
/nds bug, steel:
Durant, Escavalier, Forretress, Genesect, Scizor, Wormadam-Trash
/nds dragon, steel:
Dialga, Duraludon
Dual typing that have a 4x resistance to Ground which effectively gets to 8x with Forest's Curse added:
None!
The dual typing that resist ground 4x all have grass as one of the typing. As explained earlier that grass-types can't have Forest's Curse, the resistances can't become 8x.
Dual typing that have a 4x resistance to Water, which effectively gets to 8x with Forest's Curse added:
/nds water, dragon:
Dracovish, Kingdra, Palkia
Source
Source
That's all I could find. Please comment if anything is missing!
Hope this helps!