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 - Holds the power to play gnomes at will!Here Be DCC Monsters...General Yoros, Warrior, Str 13, Agl 8 (10), Stm 17, Per 13, Int 11, Lck 8; Law, HP 39, AC 17, R+2, F+4, W+2, band/shld, warhammer, longsword, longbow, pitchfork
Han Dee, (Weaver) Neutral Thief, Str 10, Agi 13, Stm 11, Per 11, Int 15, Lck 14, AC 13 (Leather), HP 25, Luck Die d6, Backstab 3, Sneak Silently 10, Hide In Shadows 9, Pick Pocket 10, Climb Sheer 10, Pick Lock 9, Find Trap 9, Disable Trap 9, Forge Doc 10, Disguise 3, Read Lang 5, Handle Poison 3, Cast Scroll d14+2, birth augur (Born under the loom) +1 to all skill checks (including thief skills), Banepicks (auto pick lock/disable trap, but lose 1d3 random ability loss, if a 3 then 1 pt is perm)