I see what you mean both TheDoomBug and Pingerz
With Shroom I meant his minimum is 5 if he hits on turn 1, 3 if he hits on turn 2, 1 if he hits on turn 3 (as poison does nothing after turn 4 is irrelevant on turn 4). I didn't intend for the minimum to change after it was already in effect but as you say it could be read that way. I treated the min as one would if it was damage/power reduction
"Shrink" is a better condition name I agree.
Ironically, you could read the ability of Morrigan to be round 1 is as written, and each successive round reduced by one less than the previous, ie round 4 would be -1 Opp Power, Min 6!
Maybe the best solution is to use TDB's Shrink condition to refer to quantities that reduce with time, but if wanting to affect only the min, make sure only the word Min follows the condition...
has "Growth: -1 Opp Power, Min 5" ie reduces by 1 more each successive turn, with fixed min of 5
Lear has "-1 Opp Power, Min 5, Growth: Min +1 " ie reduces by a fixed value of 1 each turn, but the min increases by 1 each turn from 6 to 9 during a battle
Rale has "Shrink: -5 Opp Power, Min 5" ie he reduces power by 1 less each successive term, with fixed min of 5
Aler has "-4 Opp Power, Min 5, Shrink: Min -1 " ie he reduces by a fixed value of 4 each turn, but the min decreases from 4 to 1 during a battle
I like thinking about ability phrasing like this but it is problematic at times