The Explorer title also requires you complete all delves by killing all bosses therein in Cyrodiil. The tricky bit is that those delves tend to have more than one boss, and the respawn is odd. Only a couple bosses are "up" at a time and which one respawns might be the exact same one you just killed, so you might have to kill the same bosses multiple times in a multi-boss delve to get the last remaining one to spawn. It helps if you use an add-on to show you where they do spawn. Also, make sure to completely loot a boss's corpse so that it disappears and respawns faster.