A grinder's game plan is to wear you down, make no errors, (probably hit few winners too), and work you around the court until you start to tire. Relentless usually sums them up in one word. They are usually very fit, and mentally very disciplined.
Plan A for you against these people, is to be steady. If you can play tight steady squash, slightly better than they can, they usually have no plan b. Their range of shots is usually limited to what they do over and over, and if this isn't working then they can be in trouble. They rely on defending well, and slowly grinding down their opponent until they are tired enough to be there for the taking. Take this away from them, and they are left with nothing.
This is easier said than done though, and you may not be able to beat them at their own game. Here's a plan b for you.
Take them out of their rhythm. they like to play in patterns, work you in patterns, and to play to set sequences of shots. They hate improvising, and hate more open broken rallies because they are far harder to defend, and don't fit into their book of plays. To do this you must
Note: 3 and 4 seemingly contradict ecahother! Use both and you'll quickly find out which works in your set of circumstances. A variety of both will often be useful, and will keep your grinder off balance!
Matches against grinders can change on the basis of one rally. Get on the end of one of their gruellers, and you can lose energy, then control, and then end up doing a whole lot of running.
Therefore, it's imperative that you focus on every shot.
Following from this, it's one of the only times that you might consider not giving your all for any given rally or period of a match. You might wish to conserve, or regain energy, by sacrificing a rally or two, to avoid falling into their web.
As you've probably worked out by reading this far, it can be a tricky conumdrum trying to find your ideal strategy. You'll find you need to stay one step ahead. Some trial and error might be involved as you try to fit the peices togther, but this can be one of the most enjoyable aspects of playing squash - solving a puzzle.
If this still isn't working then there is a simple answer to your problem. Get on the court/bike/track, get youself fitter. Now you'll last longer in the rallies and you'll be able to let your superior racket skills do the talking.