If you ever thought of how different defensive effects stack or not, read below.
A warrior was chosen as an attacker and a sentinel as defender, sometimes on the side a pharaoh dog for the aegis passive which should further reduce damage, there is class advantage and the test could be more exhaustive, but this seems to show where things stand today.
The table below shows different combinations and resulting damage, for the effects on each column:
AEGIS: Pharaoh's Dog Passive Skill, reduces the damage all allies take by 35%.
DEF DOWN: Def Down debuff, defense lowered by 70%.
GUARD: Guard Talisman used.
SHELL: Shell Talisman used
VOID: Void shield effect on
DEF: Character DEF vs ATK for that test.
DAMAGE RCVD: Damage received by the character.
All talismans used are LVL 50.
AEGIS DEF DOWN GUARD SHELL VOID DEF DMG RCVD NO YES NO NO NO 460 14821 NO NO NO NO NO 574 13721 NO NO YES NO NO 574 10291 YES NO YES NO NO 574 10291 YES YES NO NO NO 460 9634 NO YES NO NO NO 460 9634 YES NO NO NO NO 574 8919 YES YES NO NO NO 574 8919 NO NO NO YES NO 460 7411 YES NO NO YES NO 460 7411 YES NO YES YES NO 460 7411 NO NO YES YES YES 460 7411
TLDR:
Talisman mitigation effects do not stack with other talismans even if both are damage mitigation.
Talisman damage mitigation effects do not stack with characters damage mitigation skills.
Def Down debuff effect seems to be ignored when combined with/against defensive effects.
Talisman effects seem to have priority over passive abilities
Shell talisman overrides guard talisman, so it seems that higher mitigation wins priority, not taking into account additional effects (fury generation).