Ok. So say you have your group "trialmod". I'm assuming this is the lowest in your inheritance tree, so I'd guess your inherit_from would be "user". Assuming that "moderator" is your second group, the can_target for "trialmod" would be "!%moderator". What "!%moderator" means is "everything except" ( ! ) "the group 'moderator' and anything that inherits from it" ( %moderator ). I'm actually writing up a topic on this right now, so I can link it once I'm done if you still need help.
But your basics should be like this:
user
inherits: none
can_target: !%trialmod
trialmod
inherits: trialmod
can_target: !%moderator
moderator
inherits: trialmod
can_target: !%seniormod
and so on and so forth until you've reached the end of your chain. I recommend not having several 'branches' ( two groups inheriting from the same group ), it should be linear.