Exploring a very dangerous mountain castle filthy with undead, and doing well (with some very close calls). However, we ended up in a room fighting several flesh golems. It was damned close, and we were all out of healing spells. We ended up needing to retreat with a cube of force, but the golems had the door blocked. So we had essentially 16 rounds to figure something out, or face annihilation at the hands of the flesh golems (we were as I said out of healing spells, low on total spells, and low on hit points).
Now it says in the DMG that the various golems are affected in certain ways by certain spells but ignore all other spells. For example, electricity heals flesh golems, while cold and fire slows them. They ignore all other spells.
But my player (who is relatively new to the game) asked:
"If I grease the stairs, why won't they slip and fall? Likewise if I use Evard's black tentacles, why won't they grab him?"
Damn good questions!
I never worried about this much over the years because most of my players (and all of my wizard players to this point) were experienced players and simply never brought such things up.
I can see magic missiles not hurting the flesh golem, or acid arrow, or what have you. Targeted attacks where the magic affects the golem directly.
But really, why would a flesh golem not slip on stairs that were magically greased? Would he not fall into a hole formed by a dig spell? If you formed a prismatic sphere, would he be able to simply "ignore" the effects and walk through it?
Really when they say "all other spells are ignored", what does that mean? Because to me, ignoring the spell and ignoring the effects of the spell are two different things. If we take "all other spells are ignored" to mean that no other spell has any effect either direct or indirect, then we come upon illogical rulings such as golems being able to ignore areas of reversed gravity, being able to walk without slipping on greased stairs, being able to walk across the open air above a pit created using a dig spell, being able to simply walk thru a prismatic sphere unharmed. And a lot of that is nonsensical in game terms.
So how would you all rule this? How have you handled it? Don't get me wrong - I love my golems tough, immune, and deadly!

But the description as written is lacking.