For cash, SM malls (customer service desk) have the best rates

What form are your euros in right now? If it's already cash, an exchange shop is easiest. If it's in a bank account, an ATM is easiest. Look for one with a 30k peso per-transaction limit.

If you're near Glorietta Mall, there's Sanry's Money Changer on Basement 1 of Glorietta 4. Tivoli Money Exchange Location: Basement 1, Glorietta 2, Ayala Center, Makati City Tivoli Money Exchange Location: One Ayala, Ayala Center, Makati City (a short walk across the concourse from Glorietta) They usually buy foreign currency at better rates than SM. It's also where Filipinos go to buy forex

Shop around, the rates can vary quite a bit. You can check the rate at an ATM and at the SM stores in malls. Like I said in a reply to someone else, we always find better rates at local money changers, often in markets or provincial malls. The best we got earlier this year was a money changer in Imus, Cavite. The one on P. Burgos in Poblacion was decent too.

'Is it better to just withdraw from an ATM?' Yes, the ATM gives you the mid-market rate. Decline the conversion