Davao, Philippines — Communist rebels, shifting tactics dramatically, battled government forces reinforced by 2,000 marines in northeastern Mindanao. Massed in groups of 200, guerrillas mounted attacks in four villages. Previously, the rebels operated in small groups. Fatalities for the two days were the highest in recent years. Philippine officials do not consider the guerrillas' drive to oust President Ferdinand Marcos a threat. But they are known to operate in almost all of the nation's 73 provinces.