I've taken to maining Olimar and I've found that Ike's moveset is so slow that you can get 3-4 pikmin dealing tons of damage to him with a few spammed Forward-Bs. Play from a good distance from his big scary sword and keep damaging him with your Pikmin throws until there's a strong attack opening, then send him flying
One of my favourites is to wait for him to attempt a jumping attack while I have a line of Pikmin behind me, an Up-B will send him flying so you can attempt some other attacks. Olimar isn't one of the fastest characters in the game, but Ike is SLOOOOOOOOOOW, and Olimar can still beat him to the punch.
I've found getting Ike airborn and attacking from below is a good start, his Dair has a bit of a startup on it so you can usually beat him to that move, if he starts it too early you can feint to the side and use a Fair or Bair on him.
Ike is strong, just don't get hit