To anyone who keep having the !!! run away, here's how i've caught mine
attack, if missed, attack again. If successful, evade for one turn (you'll still get to attack for those who don't know.) By now the pikachu should have Either 2 or 1 Hp left, If it has 2, DON'T attack, that might cause a crtical hit and thus making the pikachu flee, use evade until it has 1 hp. If you had already gotten it at 1hp, just use a pokeball. I've never had a pikachu get away with 1 hp. *Shrug* lucky i guess. I've also had 3 !!! in a row before. I tend to make sure i click the ! in 4 second intervals to keep the chain going, superstitious but whatevs, it helps me.