Hydra = event boss (Hydra Invasion). Even the weakest Junior Hydra L1 is ~Kamaitachi-strength; Senior L3 slightly above Behemoth. Mounted. Zero-wound cavalry: L1 84.9M = t12 1.0275M @+832% (or t13 950k @+790%); L2 120.6M = t13 1.6M @+1000%; L3 198.9M = t14 1.664M @+1300%; L4 328.4M = t14 2.685M -> ~21k wounded. It gives General/Monarch EXP + stamina chips (spin Wheel of Fortune) + research stones (L3-4). RALLY NOTE: Hydra scores points once per rally, so running MANY SMALL rallies beats one big one. Doctrine (all normal/most event bosses): fight with only the single highest tier of MOUNTED cavalry, one wave, to one-round-kill for zero wounds; buff mounted attack first, count second. Skill books: Mounted Attack Against-Monster Lv4, Mounted Attack Lv4, March Size Lv4. Add an AGAINST-MONSTER debuff (not a normal debuff), e.g. Dragon Equipment Set (4); monster Def debuff caps at -50% (5x set). In a rally only ONE joiner needs the against-monster debuff -- it does not stack across joiners. Monarch Talent Lv6 'Mortality' halves wounds. Rally-join = 1 general + 1 mounted troop (loot-agnostic).
Original research source: Evony Guide Wiki
