Oak and erebus are considered strong because:
Oak: can have monks/bishops and at level 7 shield + teleport away from any fight. The only counters are stuns (before shielding) ie. graps, silence, mass charm etc. And AOE takes out the monks / bishops.
Oak struggles in 1v1 against ranged attackers (Occ, TB, autoattack build reg etc.)
Erebus can also have monks / bishops. LE can also get away from most fights, instead of shield / port there is batswarm and mist.
However assassins deal damage and lots of it. Hammer Slam, fireball, snipe... that amount of burst damage is just incredible. Beast is also the DPS king...
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