Complex sentence: He won several games against such awesome players as Mikhail Tal, David Bronstein, Boris Spassky, Lev Polugaevsky or Efim Geller: he has a plus score in the 20 games he contested against World Champions.
Split into simpler sentences: He won several games against world-class grandmasters such as Mikhail Tal, David Bronstein, Boris Spassky, Lev Polugaevsky and Efim Geller. He achieved a plus score in the 20 games he contested against World Champions.