In battle
Wonder Guard only allows super effective moves to hit. It does not prevent stat-altering moves, such as Tail Whip, from hitting or from lowering the Pokémon's stats. Also, it does not prevent damage from weather effects, such as Sandstorm.
Moves which cause status ailments, such as PoisonPowder, can still be used. In addition, the Pokémon can still be afflicted with status ailments and receive damage from those status ailments. Wonder Guard does not block Struggle from hitting, nor does it block moves whose types do not come into play for damage calculation, such as Beat Up and Future Sight. Fire Fang will also hit even if it is not super-effective. Wonder Guard may not be Skill Swapped or copied with Role Play, but it may be Traced. Aditionally, the moves Future Sight and Doom Desire are capable of striking through this ability.
Outside of battle
Wonder Guard has no effect outside of battle.