A lot of the things specifically dreamed up to counter tanks, such as AT rifles, weren't that effective. As Chris said, the thing a tank had to fear the most in the Great War was plain old-fashioned field artillery. None of the tanks of the period had particularly good armour when faced with that.
There's a cool colour plate in the Osprey on Cambrai that shows another way, if you're feeling brave. While fire is concentrated on the vision slits, two plucky Boche fellas rush a Mark IV with improvised bombs made from sacks filled with grenades. They shove these under one track, blowing it off.
Unfortunately, that didn't help. The plucky tankers refused to abandon their landship and kept Jerry away with hot fire until reinforcements arrived.
Great War Miniatures make a nice German flamethrower crew. You might have fun trying to get close enough to a British tank to put that to good use.