How hard can it be to undress a guy right?

There aren't really any rules on when to do things, or the order to do them in. Do you want to take them off? Then do it. If he's in a position where you can't, ask him or subtly nudge him to where he can.

For example, you stated wanting to remove his pants but can't because he's on top. In that case, if you can't get him off, you can actually say "I want to be on top" or "sit back" or even ask him to remove his pants himself, you don't really have to do it. You could make an attempt, show him you're unable to, and I think most guys would gladly make the removal possible (or aid in it) if they are aware of your intentions.

Perhaps you do not wish to vocalize your desires for fear of appearing assertive. This isn't really a concern considering that manually removing someone else's clothing for them is already a pretty assertive act.

Depends what you want to do with him!
I would guess the most obvious thing would be take his clothes off as he takes yours off. Ex. If he takes off your shirt, take his off.
And don't take his pants off when he is on top of you! That sounds really hard. Either get on top of him, so that you can move and get them off. Or you can do it while he is still standing.
You don't necessarily have to take everything off him to give him a hand job, if that's all you want to do. You can, but it depends on the context. If you are going to take them off, do it when you feel is an appropriate time to start giving a handjob!