Then.... Simply raise the difficulty every 5 levels?
I think enemy difficulty is easily regulated by weapon type and level. Perhaps certain weapon types would be tied to tactical choices:
Melee/Unarmed: seek cover and close in using stealth. Charge when cover unavailable.
Handgun/shotgun/energy equivalent: seek cover, close in, flank, fire from behind cover once hit probability > X
Semi-auto/lever-action non-scoped rifles/energy equivalent: Engage once hit probability > x, close in while firing. Seek cover during reload. If target behind low cover, throw grenades. If target behind high cover, flank.
Single shot non-scoped rifles/energy equivalent: Engage once hit probability > X, maintain distance. When distance
Scoped rifles/ energy equivalent: Engage once hit probability > X. Maintain cover between shots and during reload. Keep to high ground. If distance < X, retreat, seek new cover. If distance < Y, switch to secondary weapon.
Automatic weapons: lay down suppressive fire while closing in. If target behind low cover, throw grenades; if target behind high cover, flank.
Heavy weapons: Engage once within weapon range. Disregard cover, advance openly while firing.
Grenade launchers/ Incinerators/ Missile launchers- Engage once within weapon range. Disregard cover. Maintain distance > X but < weapon range. If target behind cover, attempt to flush him out with proximity explosions. If distance < X, switch to secondary weapon.