Circumstances will have a big impact on what the penalty should be, I'd think.
You could even handle a situation like this with a sliding scale -- as the players catch on to the actions of the invisible target, their penalty is lessened, round by round. And cool intuitive leaps make that happen even faster. Thus, they may start off with little to no chance of hitting an invisible foe, but get more accurate as they continue fighting, and if they play well...
Gnome Boy (a.k.a. "Jon") • DCC play-tester @ DDC 35, Feb 2011. • Beta DL 2111, 7:00 AM PT, 8 June 11.Playing RPGs since 1977
• Quasi-occasional member of the Legion of 8th-Level Fighters.—Here Be DCC Monsters—VOORAK, L, Hunter, S10, A12, Sm10, P6, I13, L15, AC 10, HP 3, Spear +0 1d8, Init 0; R 0, F 0, W -1, Holy water, Pelt, 3 iron spikes, 1d ration, 4-leaf clover (Find secret doors) (+1), Com +1
SARKLA, C, Gatherer, S9, A14, Sm14, P15, I14, L8, AC 11, HP 5, Knife +0 1d4, Init 1, R 1, F 1, W 1, Chest, Basket of veggies, 2d ration, lg sack, waterskin, torch, Seventh son (Spell checks), Com +1
TARR, C, Hunter, S14, A11, Sm15, P13, I8, L8, AC 10, HP 3, Spear +1 1d8+1, Init 0, R 0, F 1, W 1, Holy water, Pelt, 4 torch, flint/steel, 1d ration, sm sack, Fox's cunning (traps), Com
TELVA, N, Herbalist, S13, A6, Sm9, P6, I12, L10, AC 9, HP 2, Club +1 1d4+1, Init -1, R -1, F 0, W -1, Chain 10', Herbs- 1 lb., 50' rope, 2d ration, torch, Speed of cobra (Init), Com