Using Divine Judgment on a high-DPS ally can turn the tides of battle by giving your ally free reign to attack.
Kayle's attacks become incredibly powerful once she gains attack range from Divine Ascent. Play safe until then.
Kayle benefits greatly from Attack Damage and Ability Power, making hybrid items like Nashor's Tooth and Hextech Gunblade very effective on her.
Divine Judgment only grants Kayle or her allies immunity to damage. Slows and stuns can still be applied, so trap them if they're being overzealous.
Kayle is usually a fragile target. If you see her ultimate cast on an ally, switch to attacking her.
Kayle starts the game as a weaker melee champion. Try to put her behind before she ascends to her ranged form.