archus:
One thing that could work is impervious protection plus regeneration to put your liquid body back together. Sometimes the liquid or sand characters are inconvenienced by massive damage and have to pull themselves together.
That is more or less what I was going for, yes; bullets not doing much but explosives or other high impact blows would cause a level of disruption of form equivalent to injury (and of course, not having a similar level of protection for energy based attacks).
For the "lethal to non-lethal", that's actually stated in the book as an application of the Limited modifier on Immunity to Lethal Physical/Energy Damage; instead of being completely immune to that level of harm, you're so resistant that a bullet is likely to only cause a bruise as opposed to a hole, for example.
I'll look into your suggestion of "combining" protection and regeneration for the effect, though I found myself looking more at limited immunity because attacks with the penetrating modifier bypass a level of impervious which doesn't fit the feel of "liquid body resistance".