Note that you typically take damage before Guardian kicks in (I don't have Shield Charger for comparison). That would change if they implemented a shield gate, of course.
I figure something with 10 HP, something like 300 base shields, a shield recharge and an overshield mechanic, would do well. Probably either +0 or +1 health per rank. This would mean you could get to like 60 HP if you were really dedicated, but it shouldn't be ideal. If most of their abilities were duration and rather low-cost, as well as having a high energy pool, it would invite Quick Thinking without requiring it