Complex sentence: Since Sunkern is slow, defenseless and offensively challenged, making evolution the only option to effectively battle with Sunflora, which can occur by a Sunkern coming in contact with a Sun Stone.
Split into simpler sentences: Since Sunkern is slow, defenseless and offensively challenged, his evolution in Sunflora will be the only option to effectively battle. Sunkern's evolution will occur only by coming in contact with a Sun Stone.