Battle engine bug fixes & difficulty tweaks

Phase 4 now functions properly (it was horribly, horribly broken
before); also fixed a potential soft-crash bug in the AI's
item-targeting logic.

Robert now uses Desperation Slash as the Phase 4 opener, before trying
for Alcohol.  This gives the player a better chance to prevent Phase 5
from happening.  Robert's retaliation against Zombie exploitation is now
more difficult to get around, and he will counter elemental statuses
with Omni in Phase 3 after only the second infliction, rather than the
third.  Counter Stance was also nerfed: DEF and FOC are only boosted by
1.5x during Counter, and insta-kills leave the countering battler with
only 1 HP.