#$@%@#$@^% crashing Internet Explorer eating my posts. There, now that I have that out of my system, it is time to repost.
Tips for players:
Never sleep in the open. Sleep in a Rope Trick to avoid all night encounters. Since you don't have an arcane caster, too bad for your players.
CR calculation:
6 wizard
6 fighter
+1 to both for being conjoined, Attacking at night=+1-+2 CR
CR total=base CR 8, CR 9 for being conjoined, CR 10-11 for night ambush. Let's go low and say CR 10. Having the opponent fly, even though that is a class ability, pushes the CR up to 11 for your party b/c they don't have someone that can cast fly, nor are they likely to have any potions of fly, and the barbarian and fighter's effectiveness is greatly reduced.
A CR 11 vs APL 6.5? Yeah, that is tough. First thing is that they should be glad that you don't have a cleric 6/ftr 6. That would be really bad because the cleric could cast silence, preventing the lookout from alerting sleepers. But I digress...
Without knowing how you are running a conjoined creature, it is tough to give party advice. Do they own magical ranged weapons? If not, then a protection from arrows on your caster will be really sick and ensure some PC deaths. How many PCs have darkvision? That is also a factor.
If you want your guy to be able to escape, have him prep a Sleet Storm. Sleet Storm has a 40 ft radius, goes 20 ft high, and blocks darkvision. With that, he can fly away. Your abomination is vulnerable to dispel magic, so if he thinks the Druid will prep dispel magic in his couple 4th level spell slots, then he better ready action to disrupt the Druid. Also, Mirror Image is a must, as the druid would have to wait for the images to be dispelled before casting dispel magic or take his chances on accidentily choosing an image instead of the caster.
If the super twins think that dispel won't be a problem, then they can fly and bomb away with impunity.
Here is something interesting: If you party kills the fighter half first (which won't have mega magical protection), then the weight of the conjoined half will probably cause the wizard half to fall to the ground, as the maximum weight limit will be hit easily.
I have run a couple improved invisibility caster attacks vs PCs during nightime in the wilderness and it is not fun for the party. This dynamic duo can only regular invis, so it isn't as bad. Having the twins share spell effects is a no-no, i.e. it should require two casts of invisibility to get them both, since they are two, distinct creatures.
Now if you feel like being really cruel, have the wizard take mutliheaded casting and use silent and still metamagic rods to blast the party twice a round. It sound like you want to be able to show a little mercy, so you do have a few options.
Options to show mercy and cut down on PC deaths:
1) Cut down the levels to 5th/5th if the party hasn't seen hard evidence of higher levels.
2) Have him be overconfident. He doesn't go invisible and so is spotted moving in.
3) Have him offer surrender to the PCs after a few people go down.
4) Have him not target the druid first.
5) Have him forget to cast pro arrows, or have the party fight an encounter that drops magic ranged weapons if your party doesn't have any yet.
6) Have the party recover a couple potions of fly before the megamen show up.
7) Give the party one chance to get out of trouble. Perhaps the megamen sends a message to the PCs via Whispering Wind that he will leave them alone in exchange for someone. If they refuse, you can always say that you gave them an "out" and they didn't take it. Very few players (yes distinct from PCs) will allow an NPC to extort from them, unless they see evidence of clear superiority from the NPC.
Without a cleric or a wizard, the party doesn't have access to a lot of spells that would be helpful in dealing with a creature like this. I would encourage you to not pull punches when it comes time for the guy to attack, I would give them an "out".
