Interesting thread. How would your strategy be when pushing against a left hander, to get an easier opener? Say this guy loves long, fast pushes with very good depth. Mostly placed at almost the end of the table, often straight into the body, crossover or corners. Not so easy to loop those as a third ball, imho, since you might stand near the table in the beginning of the rally. And if your opponent is a decent flat hitter, you have to be careful with a half-decent loop or higher ball returns.
The thread was about a push rally, so that is where each player is pushing 2 or more balls... and neither has made a first topspin ball yet.
Your described situation is different, as your lefty likes to attack balls in the three likely spots. That is a difficult situation. You have to learn where he/she will be more inconsistent. You have to learn where their middle truly is. Sometimes it is a moving target. You have to watch their feet position and stance tendencies.
VS a lefty as a righty player, a well developed off the bounce BH is one of your best friends to pressure lefties or drive it by them or get errors to rush them.
VS hitters, it can be tough. Hard hit flat balls have only a small margin for error and they get streaky. You have to find a way to break rhythm and get them into low percentage chances, low height and placement help. Usually, they really hate extreme topspin.. but you have to have a chance to get that shot in... Heavy spin to them is like sunlight to Dracula. You have to keep probing and fighting, never give up, even if they keep landing shots, you just find a ball they are low percentage or a movement they hate... I faced a hard hitting guy with next to zero spin a couple hours ago. I was down 0-2 and 1-2 at 2-8 and 3-9 and found a way to come back point by point to win 3-2. Sometimes it is courage to do what you know will work, even if it didn't work the point before or even all match. Sometimes you discover an adjustment. TT is a game of adjustments and counter adjustments.
Below I discuss some tactics for any player you face who is not attacking you right away, but may want to attack you.
Let's say you for whatever reason are not going to attack the underspin... but it is likely opponent will. There are some things to wreck opponent's consistency. These tactics will center on variation of suddenness, variation of placement, variation of spin, variation of pace, variation of depth... sometimes variation of height (some do not attack high to BH for example.
Those 5 things I described... you can change those up... if you decide you do not attack. You do not want to feed it right to the middle depth of table or to the middle FH if they are good attacking FH like most players.
Some players do not move suddenly to the FH, so you quickly off the bounce push to FH corner or past it and watch them try for a low percentage chance.
Some players are great on their wide FH, so what do you do? Feed that wide FHG and allow them to attack cross court... where you are ready to block it by them on their BH or get a weak ball to attack for a winner. Some players do not yet topspin well on wide FH balls, but can hit if high, so a low spinny push their gets you points from errors.
Some players do not read a change of spin. You have to sell it with your follow through and make it with your touch and acceleration on a short stroke... Give a spinny one, then impact light and follow through right after ball left rubber... of just be subtle and do the same stroke... opponent reads it as under, but it is light, then pops it up for you to kill or drive... or pushes long out.
You can push to middle depth, then deep right at them and be ready to attack or get a better ball to setup something else on next ball.
You can push a foot or two from ball a couple times... then step in and push suddenly right off the bounce... you can also add a change of direction with this... very effective. Also easy to change spins this way too. hard for opponent to read it sometimes. This is a versatile tactic.
Even if you face a Rambo attacker... never get them in a rhythm. Make them change directions suddenly. Figure out which ball they want to attack that they have lower percentage, but time after time want to attack it like Dracula blood lust.