How to beat Tonberry
Final Fantasy VII Remake has two distinct opportunities for you to take on a Tonberry, the first being the Malicious Goons side quest in Chapter 14, and then as part of the Monsters of Legend in hard mode. The strategy will be mostly the same for both, although there is a stipulation on the second since it is on hard mode.
The first time against Tonberry, you will have a party between Cloud, Tifa and Barret. This allows you to have Cloud and Tifa up close, with Barret on the outside. Tonberry has a lot of health, but also takes damage quite well, though he won't be amazed by your regular attacks. The key here is to simply overpower it by constantly attacking and mixing abilities like Triple Slash for Cloud or Starshower for Tifa, depending on which person you're using.
Where the problem comes in is how can Tonberry kill you via his chef's knife attack and also in projectile form. Typically, these are his only moves outside or those that freeze a character or all three characters in place. This means if you see an icon for him about to attack, be careful. His kills with a death blow are very difficult to avoid, so your best plan here is to attack, attack, attack, and then be ready to revive them when they die by one of your two other characters, Barret on the outside being a good option.
This strategy changes a bit in the Combat Simulator version, as it has to be done in hard mode and that means no items at all. This means that if your character with the Revival Materia is killed, it will probably be over. As a result, the best strategy here is to fight with Cloud, Tifa, and Aerith. Give Aerith the Materia Revival and have her keep the same role as Barret on the outside. Focus on the Tonberry with Cloud or Tifa, which you want to use and unleash the same abilities above that are fast and deal damage in quick succession. The Tonberry should kill your chosen character instead of the others, so make sure you revive him quickly with Aerith and move on. Rinse and repeat this strategy and the Tonberry should eventually die.