Throwing grenades at a slow pace Scrake is silly at best. Do this at the Hell on Earth 6 player and next thing you feel will be the cold, vibrating steel to the face.
A single boomstick headshot at a 6p HoE Scrake will hit (1000 dual shots base damage+60% support damage bonus from level6*1.65 headshot multiplier = 2,6k while standing from a headshot) really close to the Scrake's head health(2,8k on 6player) but he'll be still at large and angry.
Throwing grenades won't damage his head health in anyway, you should pre-damage his head by taking a few (2-3) regular Shotgun shots instead.
If you just throw a few grenades to FP without trying to speedkill, he'll just avoid them in open areas and won't take enough damage to quickly fall, eventually it will enrage and loose his wrath upon the closest chump on your team
To OP; try to stand still while taking the jumpshot. When playing in a real game it would make you panic and backpedal or perform a forward jump. Try to stand perfectly still, once you sure there are enough distance within' Scrake and you.