Welcome to Librarium Online!
How do you use these units and what are they good against
grotesques are slow, strong shielders that are immune to str 6 and less weapons, they HAVE to have a hamonculus to acompany them in order to get the maximum profit. Use em if you face lots and lots of infantry thats not suited for CC like Tau or imperial guard
Mandrakes are weak and definitely arent worth their cost, you have to place 3 models on the board on the 1st turn, one of em is where the squad is hidden. They can move 6" per turn while hidden but you have to reveal em by the end of 3rd turn. They are weak but have nice cover save, good for surprising imperial guard in weak spots
Hellions are speedfreaks, the only reason you would ever take them is for hit and run for large, slowmoving armies. In the end of turn they can move 3D6 inches back without leting enemy swing at them. THeir hellglaive has a splinter rifle in it so you can shoot.