When declaring attackers for combat, are my options to...
A. either choose between declaring a single attacker or declaring a single group of attackers?
B. may I choose to declare multiple single attackers, multiple groups of attackers, and any combination thereof at once within a single combat?
Page 12 of the rulebook (http://www.epiccardgame.com/rules/) sums it up in Step A:
Choose attackers step
A. The attacking player chooses which champion or group of champions will attack, and expends them.
So to answer from your two options: A. Attack with one champion alone, or more as a single group. Either way, if any attacker can be blocked, they are all blocked by a single champion.