The start of this boss is the same for every strat. Shoot the mirror on the left with a missile or a charged beam shot and roll into the morph ball tunnel to lay a bomb in the bomb slot after the cutscene. Despite the fact that there are bomb slots you don't really have to roll into them and lay a bomb to damage Flaahgra. It is sufficient to jump right in the corner next to them or above to lay a bomb to damage it, which is a lot faster. It is recommended to mess a little around with this fight and to try out a few strats and see what suits best and works consistently for one self.
After each cutscene aim up to lock onto Flaahgra and dash to the right. Now look for a mirror nearby and shoot it either with a missile or a charged shot. By shooting the mirror Flaahgra will fall back and be distracted with the mirror for a short time. That's when you want to jump to one of the bomb slots and lay a bomb. When laying the bomb you should want to fall down into the acid, which will only deal a little bit of damage, which is less damage than getting wiped by the tentacle.
You can also stun Flaahgra with 3 missiles (if they all hit) and then jump to the bomb slot, but then you will instantly get attacked by Flaahgra after the cutscene ends. So this is only recommended for the last bomb slot.
On the last bomb slot Flaahgra will always fall down and make a wall of plants. When quickly dashing once or twice to the right one can manipulate it to go very far away giving you enough time to lay the last bomb.
If you are more confident with dashing you can try this faster strat.
Before you start the fight make sure you don't have full energy and missile count, which you can easily achieve by just wasting a missile while waiting for the room to load. This step is important to spawn a drop on the left after destroying the first bomb slot. So as soon as the cutscene is over you want to turn to the left, pull up the scan visor and dash to the right directly towards the bomb slot. You want to dash over the bomb slot and land on the right side of it to place the bomb there. This is best achieved by banding the dash forward rather late.
For the 3rd bomb slot you want to lock on to the plants blocking the door you came from and dash to the right again. This time you have to bend it more forward as your angle is a little different. Try to land on the left side of the bomb slot, morph and lay a bomb.
For the last bomb slot you can dash to the left to make Flaahgra go far left. Now use Flaahgra as a dash point to dash directly to the last bomb slot. If you don't feel confident with the dash then you can do any of the easier strats.