With 95 units of knockback-based armor, it can be hard to launch Giga Bowser. Hard, but not impossible. These are all of the attacks that I could find that can launch (or at least flinch) him.

The effectiveness of counters is based on incoming damage, that is it is based on Giga Bowser's damage. On lower difficulties he deals less damage, thus counters may not reliably deal enough knockback. For example, on Very Easy you may only get his forward smash and forward tilt to work; on Hard even his jab works.

0:00 start
0:26 Mr. Game & Watch - Judge #9 (Side B)
0:36 Cloud - Limit Break (Down B)
1:01 Hero - Hatchet Man (Down B)
1:06 Hero - Up smash
1:15 Kirby - Hammer Flip (Side B)
1:27 Marth - Counter (Down B)
1:31 Lucina - Counter (Down B)
1:35 Roy - Flare Blade (Neutral B)
1:41 Roy - Counter (Down B)
1:52 Chrom - Flare Blade (Neutral B)
1:59 Chrom - Counter (Down B)
2:14 Ike - Counter (Down B)
2:19 Corrin - Counter Surge (Down B)
2:22 Little Mac - Slip Counter (Down B)
2:25 Shulk - Vision (Down B)
3:08 King K. Rool - Gut Check (Down B)
3:12 Joker - Tetrakarn (Down B)
3:19 Mii Swordfighter - Blade Counter (Down B)
3:48 Item - Hothead
4:07 Item - Death's Scythe
4:50 Honorable mentions
4:52 Jigglypuff - Rest (Down B)
4:56 Incineroar - Forward smash with Revenge
5:02 Lucario - Forward smash with max Aura
5:07 Byleth - Aymr (Down B)
5:10 Hero - Forward smash with Oomph and Psyche Up