Modern grots have a flesh gauntlet and a close combat weapon. They can swap any close combat weapon for a liquifier at +15 points, but you should basically never do this. Liquifiers are horrible as of the 7th edition codex, and I went from fielding like 6 of them to fielding 0.
The consensus seems to be that aberrations are really only necessary if there is another character in the unit, so he can take the challenge. You can arm him with quite a few choices to replace his close combat weapon but really it comes down to scissorhands (+10) or an agonizer (+25). You can find some debate about this elsewhere although I personally prefer the scissorhand for killing walkers with the occasional rending hit.
Rending isn't reliable, but I've actually run in to an issue where grot squads have been caught up on something as minor as a ~35 point armored sentinels that the unit can't harm without the upgrade(HP2 but front armor 12). I've never had too much trouble with the 3+ armor an agonizer would counter though.
Then again if you aren't running coven/fearless grots, you may want to take advantage of the "our weapons are useless" rule and avoid the scissorhand. In general I'd just try to keep grots away from anything resembling a walker unless it's tau.