TL;DR: If Terra's 3 gets destroyed by enemy basics in its healing state, it turns into the damage state
There are currently 2 main problems with Terra:
She needs 2 abilities to properly clear, which means her lvl1 is terrible (which is really important in the current meta)
Her Monolith is easily destroyed at all stages of the game.
What I would suggest is that instead of her 3 disappearing when it gets destroyed by enemy basics, have it turn into the damage field first. Heal field – destroyed – damage field. Basically just as if she shattered it herself.
What would this do? She can now justify leveling her 3 first, and place it in the middle of the enemy wave. Now either the enemy leaves it up, allowing terra to go up close with the additional sustain she gets, or the enemy destroys it giving her additional clear and zoning.
For later stages of the game it doesn't have too much of an influence, it simply makes the ability a bit more reliable. You can use it to block shots from the enemy adc, and if he destroys it he punishes himself giving you a slight edge, instead of rendering your spent cooldown completely pointless.
If this change turns out to be too strong for some reason, there's enough options to balance it out. Maybe have it only turn into the damage field, but without the root that comes with shattering it intentionally. If this is still too strong, have it only apply the instant damage it got on the recent patch if it gets destroyed, without the lingering field.
I think this would give Terra some viability back, in form of reliability instead of simply brute-force boosting numbers until she's too strong again.