>>16705183252 Atk Parental Bond Mega Kangaskhan Return vs. 252 HP / 0 Def Barbaracle: 108-127 (31 - 36.4%) -- 66% chance to 3HKO
(108, 109, 111, 112, 112, 114, 115, 117, 118, 120, 121, 121, 123, 124, 126, 127)
252 Atk Parental Bond Mega Kangaskhan Power-Up Punch vs. -1 252 HP / 0 Def Barbaracle: 171-204 (49.1 - 58.6%) -- 98.8% chance to 2HKO
(171, 174, 177, 177, 180, 183, 183, 186, 189, 189, 192, 195, 195, 198, 201, 204)
252 Atk Parental Bond Mega Kangaskhan Power-Up Punch vs. -1 252 HP / 0 Def Barbaracle: 171-204 (49.1 - 58.6%) -- 98.8% chance to 2HKO
(171, 174, 177, 177, 180, 183, 183, 186, 189, 189, 192, 195, 195, 198, 201, 204)
Conversely.
+2 252 Atk Barbaracle Razor Shell vs. 0 HP / 0 Def Mega Kangaskhan: 211-249 (60.1 - 70.9%) -- guaranteed 2HKO
(211, 213, 216, 219, 220, 223, 226, 228, 231, 234, 235, 238, 241, 243, 246, 249)
Even a crit has a rougly 50% chance of OHKO.
Assuming it doesn't sucker you. Barbaracle can take 3 hits max, even less once shell smash goes. Even with White Herb, there's no way Barbaracle could stop M-Kanga.
Source:
http://pokemonshowdown.com/damagecalc/