In buddhism one of the four sublime states is equanimity, so the adjective I'd suggest is equanimous, (which doesn't quite trip off the tongue).

Also of relevance, for interest: Akkosa Sutta: Insult

In buddhism one of the four sublime states is equanimity, described as:

Equanimity is a perfect, unshakable balance of mind, rooted in insight

Thus, the adjective I'd suggest is equanimous, (which doesn't quite trip off the tongue).

To learn more, see: Akkosa Sutta: Insult