Why is that laughable?
I'm making the following (reasonable, I think)assumptions:
1. The character in question is in a party of 4-5(in my experience, the average party size is actually 6-7, due to the 'more players than DMs' effect, but I'm going with 4-5 here)
2. The hypothetical combat in which this is happening is an on-level encounter(at level 2, the hypothetical thief's level).
3. It is built in a fairly generic way, ie, a number of standard enemies equal to the number of characters in the party, and of the same level.
Given those fairly base assumptions, the Thief can drop one or maybe if he's really lucky, two of those enemies. Thus, 3-4 enemies left. The encounter has become easier due to his use of resources, but will still take some beating. That sounds about right to me.
Do you LMAO because you put out encounters weaker than that? If so, they're probably getting mopped up quickly anyway, so I'm not sure where the LMAO comes in.
Do you LMAO because you put out encounters harder than that? If so, they're going to still be a challenge, even after the player blows his resources(and hard encounters basically invite the blowing of resources anyway).
I'm really just trying to figure out what you're getting at here.
As an aside, I'm active in PbP on these boards. If you ever do want to DM for me sometime, I'm game.