By this stage you will often have some idea about how your opponent approaches the game, even if you have never played with them before. The stacks are still fairly deep, so you should continue to take notes and try to find weakness that you can exploit, but they are not as deep as they were at the beginning of the tournament so you will have to be a little more careful. Remember that you can still three-bet and fold, but it is pretty expensive and you shouldn’t be three betting with a lot of hands you aren’t willing to go all the way with unless your opponent is a huge wimp and folding to your reraises nearly every time. It’s better to defend your big blind by calling if his raises are minimum, or playing fairly tight and occasionally three betting if he is raising to 3x.
You may have found that your opponent folds his big blind too often, in which case you should be raising nearly every hand. Many solid players prefer to raise 90% in this situation rather than raising every hand, because the occasional walk in the big blind encourages your opponent to continue to play too tight.
If your opponent is very loose and reraising often, you can tighten up quite a bit, raising only 60% of hands against the wildest players. The correct 60% and 90% ranges are below, along with a 75% range to raise against solid players.
It will still be important to play well after the flop in this stage of the game, making continuation bets on every flop if you were the preflop raiser against all but the wildest opponents. If you defend your big blind, you will want to check raise any pair and any draw with 8 outs or more. Below 30 big blinds this check raise will leave you pot committed in almost every case. If you think your opponent is likely to continue to bet you may want to slow play your strongest hands, usually top two pair or better, though check raising with these hands is a fine way to play them as well, especially if you have raised a lot of flops recently.
How often you choose to defend your big blind depends on the player across the table from you and what their raise size is. You should be tight if they are raising to 3x, tight, playing well after the flop, and will often shove all-in if you reraise. If all these conditions are met, you can defend as tight as 24%, but this is extreme and you will not often find players who are raising to 3x at this stage, are willing to go all-in preflop often, and play very well after the flop.
Against weak opponents who play badly after the flop, and min-raise, you can defend very light because it isn’t costing much and you have a good chance to stack them if you hit your hand. Against the weakest min raising opponents you could defend with a range as wide as 51% if you are comfortable playing out of position after the flop.