I thought that charizard was the hardest of all to catch(dang fireballs).
It was hard, but the "mission" with the challenges was harder for me... In the last mission if you have two Gengars (for Suicune and Entei) it's a piece of cake... And strangely I found that Psyduck is harder to catch than Entei XD (it required 32 loops O_O, and it escapes after like 5 seconds)