What is Shield Gating?
When shields deplete to zero, you do not continue to lose health and instead become invincible to all damage for a short duration known as the shield gate. The duration of the shield gate is affected by the highest shield value reached prior to depletion, ranging from a minimum of 0.33s below 52.5 shields, to 1.3s at 325 shield to a maximum of 2.5s above 1150 shield. In essence, as long as you have any amount of shield, you cannot be one-shot by any amount of damage (except via toxin damage that bypasses shields).
We can take advantage of this 'shield gate' to stay alive at any level by regenerating shields on demand through mods such as Brief Respite and Augur Set Mods and warframe abilities such as Pillage and Condemn. Sentinels can also consistently and passively regenerate shields through the combination of Guardian and Manifold Bond. Alternatively you can also wait for passive shield regeneration to kick in after 4s of not taking damage, which you can bridge through external i-frames such as Rolling Guard (3s) and Vazarin Protective Sling (5s). Catalyzing Shields is an optional addition that improves shield gating effectiveness in certain situations by reducing the maximum shield capacity but capping the maximum shield gate duration to 1.33s. This allows abilities used alongside Brief Respite/Augur Mods to generate more effective i-frames per cast.
CROWD CONTROL is absolutely essential for comfortable play in a shield gating setup since it significantly reduces the frequency of shields breaking in the first place. The best forms of crowd control also affect Eximus through overguard e.g. Shooting Gallery with Muzzle Flash augment, Breach Surge.
See here for a practical video example (slightly outdated): https://www.youtube.com/watch?v=_SouSM_TmWU