How can a 15th level Wizard survive an assassination attempt that he knows is coming, but not when or how?

Question

Gregor, a 15th-level Wizard, has angered someone that he knows will try to assassinate him, a 17th-level Fighter named Vadim. Vadim is a crazy person and has no friends, but possesses a custom artifact that wards him against divination (targeting him or his possessions) meaning his location cannot be simply found for the ol' gank squad murder-rampage to be an effective solution.

What steps can Gregor take (assuming that he doesn't want to live in a box floating in outer space or something for the rest of his days, he has business to take care of in the regular world, wants to expend spell slots occasionally, etc) to survive Vadim's eventual assassination attempt?

Vadim may have some means to stop people coming back after they die. It's unknown. Thus methods that involve letting Vadim kill Gregor and then returning Gregor to life are potentially useful but not necessarily the best - methods that involve not dying in the first place are superior.

Answer 1

If Vadim attacks Gregor, he could plausibly make six attacks (three per action, with Action Surge). If Vadim has a +3 greatsword and the Great Weapon Fighting style, each attack could be at +14 to hit and deal 16.33 damage on average, for a total of 98 damage, before accounting for crits / crit misses.

Vadim can actually do much more than this by making use of class features. For example he can use Great Weapon Master to take +10 damage and -5 to-hit, and then use Battle Master Fighter with the Trip Attack maneuver to give most of his attacks advantage, or the Precision Attack maneuver to add +1d10 to any attack roll that misses.

Worse, Vadim could get a surprise round on Gregor (for example by drinking a potion of invisibility), and then do his entire attack routine twice. Gregor probably has 107 hit points (assuming a 16 CON and base 4hp per level), and AC 16 with mage armor, so if a combat happens, Vadim has a very good chance to kill him before he can act.

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Luckily there's a spell combination that handles this problem.  Gregor can cast contingency with resilient sphere.  The safest trigger is probably something like "I take damage", or possibly "I have fewer than 100 hit points".  The resilient sphere blocks any attack that Vadim could make, and gives Gregor time to teleport or plane shift to a safe location.  Gregor can stay in his safe location until he can recast his contingency resilient sphere and then resume city life the next day.

If Gregor wants to stay and fight, a good first move would be to stop concentrating on the resilient sphere and then cast forcecage, choosing the variant that produces half-inch holes in the cage.  This spell offers no save and no way for Vadim to escape, though Vadim may still be able to fire arrows through the half-inch holes if he remembered to bring a bow.  Gregor can take cover behind a building and use his choice of damage-over-time spell (perhaps wall of fire or sickening radiance) to finish Vadim off.

Alternatively, Gregor could use the solid forcecage with no holes in it, which will hold Vadim long enough for law enforcement to arrive on the scene and deal with him.  

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Finally, please note that, if Gregor is worried about being killed, he should cast clone.  It's probably not needed for this exercise, but it's a good general precaution that everyone should take if they have the ability.