Szalvinzir has 5 phases:
1 Szalvinzir, the Arena Champion (Lvl: 100)
2 Szalvinzir the Great (Lvl: 101)
3 Szalvinzir the Mighty (Lvl: 102)
4 Szalvinzir the Revealed (Lvl: 103)
5 Stone Memory of Szalvinzir (Lvl: 104)
Phase 1: Throughout the fight, spawns a large number of "a hissing serpent" (Lvl: 75) which are very weak with low HP. If you burn him down before any serpents spawn, you will get the I Love Snakes achievement.
Phase 2: Throughout the fight, spawns "a bladestorm" (Lvl: 98) one at a time. We ended up with 5 or 6. They hit for up to 8000.
Phase 3: Occasionally Szalvinzir will lock HP and a trap will spawn. Drag Szalvinzir over the trap to resume killing.
Phase 4: Crystal form. Nothing special.
Phase 5: During the fight, spawns "a shissarian nightmare" (Lvl: 98) one at a time. 4 total. They hit for up to 10000.
The event can be reset by moving outside the arena.