ToME: the Tales of Maj'Eyal

Dual Strike: 15 stamina, 3/4/6/7/8 duration, 6 cooldown

Stunning Blow: 8 stamina, 3/4/6/7/8 duration, 6 cooldown

Shield Pummel: 8 stamina, 2/3/4/4/5 duration, 6 cooldown

Obviously Shield Pummel got the short end of the stick here. Buffing duration to 3/4/6/7/8 to match Stunning Blow should be good.

Unsure how long this will be relevant for, but yes

Alternative idea to make Shield Pummel different

Change it so that each of the Shield Pummel strikes has a chance to stun/add to Stun Duration, similar to Noggin Knocker on Skirmisher and bump up the cooldown duration to compensate for the increased stunning potential. That will set it apart from the other skills which are also decent for committing damage to enemies.