CardsplainThe fine print
looks at your creatures at set moments. Nasty Little Rabbit says if. It looks when the beginning of combat step starts and again when its ability , so if the opponent kills your only 4-power creature in response, the Rabbit gets nothing. Wargling and Wilderland Scrounger say while. They look once, as they are declared as attackers. If the 4-power creature leaves after that, the ability still resolves. Wargling counts itself, so Smaug's Fury (+3/+0) on a lone Wargling turns its own ferocious on, included.
Timing a pump. Smaug's Fury cast during the beginning of combat step is too late for the Rabbit, which has already looked, but in time for Wargling. Cast it in your first main phase to get both.
Bite and fight. Quarrel is a bite spell. Your creature deals damage equal to its power and takes none back. Troll Negotiations puts two +1/+1 counters on your creature and then makes it fight, so both creatures deal damage equal to their power at the same time. Both read power as they resolve, so a pump cast in response to your own Quarrel counts. If your creature is gone or no longer a legal target, no damage is dealt at all. If only their creature is gone, Troll Negotiations still gives yours the counters, but nothing fights. Fight damage is not combat damage, but works on any damage, so Attercop destroys whatever it fights.
Damage adds up until the turn ends. Damage stays marked on a creature until the cleanup step, so 4 combat damage plus Pinecone Strike's 3 kills a 3/6. Damage never lowers power, so a 4/4 with 3 damage on it still turns ferocious on. A -1/-1 effect lowers toughness, so that 4/4 if it gets -1/-1. Pinecone Strike's " it instead" lasts the whole turn. If that creature dies later in the turn, it is exiled.