Could You Beat A Grandmaster If...

...you have one of the highest scores on Tactics Trainer, but never study positional chess?

No.

NO!!

The previous two comments are far to short to capture the nuances of your question. The more precise answer is "You gotta be freaking kidding me".

Has somebody made that claim?

What does it mean "but never studied positional chess" Even a 1000 rated player has some positional knowledge.

Chess.com tactics trainer is not the most accurate display of chess tactics rating. Having GM tactical skill won't make you a GM, but might make you strong enough for a title.

It's a hypothetical question. Pretend that someone was locked in a room, and was only allowed to do Tactics Trainer, and, o.k., was given the most basic rules of the game, like promotion, castling and en passant. Could this person, after scoring over 3000 on Tactics Trainer, beat a Grandmaster?  It is said that chess is 99% tactics. 

I'm assuming he means things like pawn structure, corresponding squares, maybe Lucena and Philidor? Though i guess some of those could be considered tactics too.

Also, is the OP thinking about the claims that chess is mostly tactics? While tactics is important, i think other chess game knowledge is at least as important. 

Just look at ejl34's game scores and you have an answer to your question.

Online chess: 

Here is the problem with tactics trainer. You are already alerted to the fact that there is a tactical combination. You already know that you have a win. In real chess, you have to get to that position and recognize it. The Hallelujah  Choir doesn't come down from on high to announce it.

Tactics trainer has its benefits, don't get me wrong, but a 1200 player studying it 8 hours a day for a year won't become a 2000 unless they are doing other studying and playing.

So tactics trainer won't even guarantee you are a good tactics player. And the quote that chess is 99% tactics was from a German master from the turn of the century who was known as Richard the Fifth because he finished many tournaments in fifth place. GMs after him have quoted him, but not in a serious manner.

I don't think I can.
having said that I have won against a player otb 2 hours game that has won against a player that won a simul against a GM and drew against another. I think that player is sandbagging or just don't care about winning and losing a lot since he has a rating of 1400 and beats players with up to 2000 in rated games. 

I could beat a grandmaster if he had been up for 48-hours straight, and he also had Nickleback at ear-busting volume blasting through earbuds. But just to be sure, he also has to have three cups of ice in his pants.

Or plays using an engine against GMs.

A large percentage of chess positions don't have any tactical shots, if you don't know the basic principles of strategy you will be wiped out by any GM.

the problem with chess.com tactics trainer is its more of an intuition trainer as the short time per puzzle rarley gives enough time to calculate all alternatives properly. I prefer the tactics trainer on chesstempo.com has better tactics and you have plenty of time to calculate, in my opinion calculation is a very important part of being strong tactically. It is one thing to recognise a tactical pattern but another to calculate accuratly. I think chess.com tactics trainer is good for helping you recognise patterns but not enough time to calculate. Also some of the advanced tactics courses on chess mentor are pretty good.

How would that be possible in a otb simul. the player only plays otb games.

My bad. I can't read. ^^

I disagree. During a chess.com deathmatch on chess.com tv, IM Danny Rensch and IM David Pruess both saw a blunder on the board at the exact same time, in about 2 seconds. One of the reasons for tactics training, the way I understand it, is to instantly recognize a mistake by your opponent, and then cash in.

Are some tactics going to take a deeper, longer look? Sure, but the ones with tactical vision in the high 2000's, up to the 3000's here, can already see fairly deep in very little time. I would imagine that they first get a feeling that something doesn't look right about the position. You don't need to always be alerted that there is a tactic on the board to see it. Even at my low tactics rating, forks and other basic tactics pop into my head during 15 10 games.