Except you are extremely excellent tactician, you should stick on medium Battle difficulty...
A unit of peltasts meleeing 3 units of your hoplites unto death? I have a hypothesis:
1) This was Peltastai Makedonikoi, they are elite infantry, so don't get surprised... They can chew even Hypaspistai...
2) You play on hard, the enemy got +4 attack, and Peltastai are good melee troops anyway... didn't get surprised if your Hoplitai Haploi get meleed to death... (I don't believe if it was Standard Hoplitai)