I just a hydra fight that felt suitably challenging and a little scary (A CR 8 in favorable circumstances against a party of 5 level 5s should have felt threatened), but they met it in melee and no one moved the whole fight (except the ranger's hawk that had fly-by attack).
The hydra is a good example due to it making a number of OAs equal to its heads. While all monsters benefit from encounter design, this is one that
especially benefits.
I ran a pyrohydra – more heads, fire-breathing, fire resistance, head regeneration stopped by cold or poison (instead of fire) – against a party of five 10th level PCs in a PbP game. The key to making it interesting was that I placed the pyrohydra in an oasis pool. It ambushed the party, attacked from the water with a combination of Reach bites and grapples. Then it breathed fire erratically, causing the water to heat up, and dragged the grappled victim under into the near-boiling water. The PCs managed to kill it after two rounds, but they were using the Climb on a Large Creature action, ranged attacks, weighing the cost of taking ongoing fire damage to save their grappled friend, and making heroic leaps to close the gap.
This made it feel a bit like the Watcher in the Water from LotR.
A similar thing could be done with the regular hydra. It stays in the water, 10 feet from PCs, biting and grappling. The challenge for melee-oriented PCs becomes figuring out a way to close to melee with the hydra without entering the water (where they could suffer disadvantage on attacks). The challenge for spellcasters becomes figuring out a way to pressure the hydra to quit its advantageous position and enter melee (e.g. an ongoing damage spell centered on the hydra).
Just looking for some ideas. I'm tempted to remove opportunity attacks. I'm tempted to reintroduce acrobatics checks to avoid OAs or diverse Move Actions. I'm definitely going to be rethinking encounters more as well.
What are your thoughts?
Pathfinder 2 and D&D 5e seem to arrive at a similar place in regards to mobility & teamwork through different rules. 5e's approach is to encourage tactical teamwork through the threat of being opportunity attacked.
Say the wounded cleric wants to move without being opportunity attacked by the hill giant, in order to cast
cure wounds on the distant rogue PC (and the cleric also wants to be out of melee with the hill giant). The other party members need to figure out a way to lockdown the hill giant's ability to use its reaction (e.g. by casting
chill touch or incapacitating the giant) or else provoke it to expend its reaction on, say, the fighter or barbarian engaged in melee, so that it has no reaction to use on the cleric's initiative count.
If they're not able, or not willing, to implement a plan as a team, then the cleric is faced with a dramatic choice: risk their own well-being (i.e. being opportunity attacked) to heal the rogue, or Disengage safely and postpone
cure wounds but risk that wounded rogue might not last another round.