There is definitely something wrong with this planet. within its atmosphere and on its surface all technical equipment fails to work. communication is impossible and your ship has crashed. The planet is inhabited by a lost human (or alien) group whose complex religion is based around a great mountain spirit. This spirit is in fact what is causing a world wide "jinx" effect. This entity lives in a large temple complex surrounded by worshiper and guards. the only way off the planet is to assault the temple and kill the entity.
rules are simple. no technical equipment works. no gun, no power weapons, no shields, nothing. this should applies to tyranids too (don't ask how). also all psychic powers too.
each sides leader creates am anti jinx effect around them. all equipment works as normal within 6" of the leader.
goal: kill the enemy leader.
as the governor of a hive-world you find it difficult to keep order at the best of times. things have been getting worse since a madman started a new religion based around the premise that a vast floating pudding would appear to make life better for everyone - the people have stopped working, even the military has been affected and you can't rely on your own staff anymore. yesterday you caught your own mother reading a pamphlet entitled, 'The Pudding is coming ~ ten reasons why you should believe'. today the prophet of this insane religion , the Rev. Jeronimo Kipling, will be conducting a whistle stop tour of the under-hive. you have positioned a special unit to attack and kill him. the troops are dressed in civilian gear as you intend blaming the whole thing on a rival religious group (possibly the Anadentist or The Church of the Lucid Shirt button). the target will be surrounded by the usual bodyguards and mobs, and his vehicle may be well protected.
rules: no weapons over S8 and no vehicles. decide who will be the attacker and who will be the defender. ideally this would work best with two guard armies but make do with what you have. place three rhinos in a line in the middle of the table long ways. the defenders leader is placed in the middle rhino and now treats it as his transport. any other squads can be placed in the other two rhinos (use common sense). the remainder of the defenders army is placed with in 6" of the rhinos. the attacker may set up any were along booth long table edges up to 6" in. the attacker starts first.
goal: defender must have his leader leave the table. the attacker must kill the defenders leader.
a rebellion on a feral world has established a new anti-Imperial government. Pro-imperial rebels have taken to the hills where they are trying to persuade a stone age tribe to help them. only by helping the primitives overcome the fearsome monster that has destroyed their village can their support be gained. the monster is tracked down to a complex cave system where it lives with other of its kind. this cave would make a good headquarters once its current occupiers are evicted.
this ones an easy one
the monsters can be tyranid carnifexes with net to no range weapons. to simulate surprise have them all deepstrike. any that are killed deepstriking just get held over to next turn. to simulate a cave system just toss down a bunch of hills and make them impassible.
goal: kill all monsters.