Genesect is pretty slow with its 99 base speed. Scarf Terrakion in the sand,Keldeo in rain,and Heatran can all cointer Genesect with ease. Chansey with eviolite and Gastrodon for Genesects lacking Giga Drain also work. For countering Jirachi Jellicent works for most Scarf variants or those that choose Body Slam over Thunder Wave. Landorus is naturally faster and can revenge kill Jirachi. Heatran doesn't give to much care to Jirachi either as long as it isn't the rain variant with Substitute and Calm Mind.