We don't need to come up with other ways to destabilize the Abu Merci as Mason has already given us a reason. If it doesn't have propellers, then it can't move. That's pretty destabilizing by itself, and breaking an engine won't force it to the ground. Even damaging and risking the destruction of a very valuable engine when we can get the same result just by damaging a rather cheap propeller or even cheaper balloon is simply wasteful.
As to the ease of taking out the propellers or balloons, how can it be risky going onto the top deck? All you have to do is pop your gun up through the hatch and shoot out all the balloons. Its not as if they're hard to hit, especially with a shotgun. I also have to say that balloons don't care how puny your bullets are, they cannot be armored or else be completely useless and so are really easy targets.
I also notice you're ignoring of my other point, so I'll ask perhaps a more pointed question. Why put me, a pilot, out to use explosives, a field of knowledge that pilots, especially when I'm essentially a truck driver, tend not to have? Although I may or may not have a knowledge of engines, would not a mechanic be more useful in this adventure, however ill-thought out?
Let's see, that image, what little it was, showed quite a bit of crap between the deck and balloons, plus deck-mounted weapons,
The propellers would likely have some kind of protection near them to head off any such attacks,
The damn thing has
planes that would shred our blimp if we left it exposed too long,
I would have to leave a person with you unless you can somehow operate a blimp and a machinegun at the same time,
And breaking an engine would slow, if not stop, all of it's motion. If that thing relies not only on balloons, but thrust as well (Very likely, that thing looked too heavy for what little ballon space it had!), it may just be forced to land, if slowly. It would certainly get within range of Machu's guns very quickly.
Also, I am not asking you to use explosives... I am asking you to damage a piece of equipment that is likely to be unarmored. Bullets from a shotgun could wreak havoc on it, in all likelihood. I'm fairly sure a "Truck Driver" can fire a shotgun.
And Zako is with me. You are with me too until you break off to go to the engine room.