on VH battle difficulty, i found that grenadiers were the only troops that even had a chance against enemy line infantry. To maximize casualties, wait till the enemy are bunched up, and exchanging fire with another of your units. you can easily cause massive casualties amongst the bad guys by charging up to nade range and letting loose some volleys back to back. grenadiers seem to die a little slower than regular line infantry as well. Not to mention they don't run away so easily.
IMO anyone that thinks grenadiers don't work are simply using them incorrectly. The battles I was using them in (and losing), the tall boys were inflicting 100+ casualties per 60 man unit, almost exclusively from grenades.