It's better to use scouts to take out the Matrons. And the Hydra can even block the Matrons themselves. Simply standing there and pressing "Defense" with the goal of holding out until the hero's turn won't work either. Because you've given up a weak unit, and then high-initiative units will attack for free until it's the Hydra's turn. And her stats are very weak. How can a Hydra counter a Druid? And in general, this is a problem for all melee monsters. On such a battlefield with ranged attacks, they are easily taken down.

Okay, I'll answer each point:

Regarding the scout – doesn't the fact that it will be taken down with one hit matter?

Regarding blocking the Matrons themselves – do they need to run anywhere?

Regarding "giving up a weak unit" and then attacking – this applies to neutral creatures. Where will you find one in PvP? And there are also high-initiative units on the side of the Hydras and Matrons, and the Matrons can easily reduce the initiative of someone particularly agile by half. I've used simply pressing "Defense" repeatedly because, as I said, it needs to be until the HERO's turn, not the Hydra's (and with an Expert Sorcerer, the initiative is approximately 13, I've checked empirically).

Regarding countering the Druid – I don't see how the Druid is better (in PvP, of course; as neutral creatures, Hydras are definitely easier to defeat). 8 Elder Druids (weekly growth from a castle) = 264 HP, 6 Chaos Hydras (weekly growth) = 600 HP + regeneration. So, which is better?
(I'm not claiming that Hydras are good in all cases, but definitely for a mage hero.)

Regarding the problem of all melee units – yes, as neutral creatures, they are easier to defeat than ranged and caster units. But this has been the case in all the Heroes series. Simultaneous attacks also didn't solve the problem because "single-damage" attacks and spells with 1 damage were used extensively. In my opinion, the only way to solve it is to introduce additional retaliations (like the Griffin). The higher the level of the monster, the more retaliations it has; for ranged and caster units, this number is lower than for melee units.