You can give it damage reduction through a few abilities to negate a hundred percent of the damage. One of these I believe is hardened skin, another defend, and another mana shield. Mana shield is good because you can reduce a percentage of the damage, rather than a high numerical number.
Another way is by changing the units classification through triggers. This would allow the unit to attack but to not be targeted ( I wasn’t sure whether your comment was referring to ethereal ability or classification). Ward classification generally gives enemy units less priority to that unit. You can also give it a classification through trigger of invulnerable or ancient.