Grover is NOT CC immune during his ultimate, however if he is stunned/chickened it will not interrupt his healing AOE.

As a frontliner, try to bait out Recharge by using secondary shield abilities (i.e Safe Travel, Bowling Ball, Protection ) and activate your main shield after his ability ends to avoid him siphoning it.

Viktor's Barrage actually spawns explosions on the ground instead of dropping them from above. Moving backwards while he is locked on to you will make them hit your shield if you get the center of the blast off you and in front of your shield.

When trying to counter King Bomb with Immortal, make sure to activate your ultimate only after his voice line is over and Bomb King is rolling in your direction, or else he can wait it out and detonate after your ultimate has expired.