Q. You've got a Plagued Rusalka
on the board, which is 1/1 and has the ability to sacrifice a creature to make another creature -1/-1. Your opponent is attacking with two 1/1 creatures, and you want to kill them both. How would you do it?
A. Use the ability to sacrifice itself after damage is counted (or "on the stack") but before it's dealt (or "before it resolves") targeting the creature you're not blocking. If you sacrifice the Rusalka before declaring it as a blocker, one of the attackers is going to go through unscathed. If you use the ability before damage is counted up, you're not going to hurt the attacker you've chosen to block. And finally, if you try to do it after the damage is dealt, you're going to have the problem of the Rusalka already being dead.
Q. You're attacking with a Golgari Brownscale
(2/3 with abilities we're going to ignore) and your opponent blocks with an Agent of Masks
(2/3 with abilities we're going to ignore.) You've got a Darkblast
in hand - an instant that gives a target creature -1/-1. At what two times would playing this spell be the best?
A. Before Combat Damage step or after combat completely. Ordinarily it wouldn't matter - you'd kill the Agent of Masks no matter when you cast it. Even if the Darkblast isn't technically doing damage, if it lowers a creature's toughness to two, and the creature has already taken 2 damage, then it will die. So, if the creature will die, why should it matter when we cast it?
Imagine your opponent is holding onto a spell to pump up their creature (such as that 'Gather Courage' mentioned earlier.) If you play your spell during combat, they can come back and cast their spell in return, which in this case would turn their Agent of Masks into a 3/4 (which would kill your Brownscale.) If you wait until after combat, you can see whether your opponent has a pump spell. And if they don't, the Darkblast is just as lethal afterwards.
On the other hand, if you're worried about your opponent doing one damage to your creature somehow (such as them having a Viashino Fangtail
out, which can tap to do one damage to target creature/player) you should use the spell before damage is counted. Otherwise, the Agent of Masks will be doing two damage to your Brownscale, and one more at any point will kill it off.
Knowing the steps here was essential for getting the most out of the battle. In the first situation, using the spell wrong would result in your opponent not losing a creature (either by not taking four damage or simply choosing not to chump block.) The second situation, you would only kill one creature where you could kill them both. And the last, you could lose your creature and waste a spell without doing anything to your opponent.