In magic the gathering can I tap a creature at the attacking/blocking phase?
You can always ask your opponent to "rewind" phases if they attempt to skip through them.  According to the rules, each player gets a chance to act within all phases, however, many players choose to skip through the phases in interests of saving time. A player is never allowed to "rush" you through a phase. They will need to reset their creatures to how they existed before attackers were declared.

In this particular case, if they move to attack step before you get a chance to respond, I would politely inform them that I wish to cast a spell during the "begin combat" phase.

For your side note: there are many cards such as Celestial Flare that effect attackers or blockers (scroll down to the bottom for example rulings that I believe you will find helpful)