Hammerdin, I'd say definitely too slow for competitive Rune Finding at Cows. It takes me around 5 minutes to clear the whole level (includes getting the leg), although I'm still fairly inexperienced in it, but I doubt I can drop it that much. My Hammerdin is pretty much fully decked, I can't really get better damage any more unless I play with full stash and use couple of more skillers (which I have not even found yet), but that doesn't seem to really make it or break it. The fact that they are 33% Magic resistant really shows.
With a bit more experience and practice at herding, I might be looking at 250 runs during 20 hours. I wouldn't be surprised if a godly geared Javazon with a hands of an experienced Cow runner reaches 500 runs or more. On the other hand, I got like 4 runes per run, so if that's the average, I'll need to find a qualifier every 200 runes or so to get 5 and a "complete score". And I estimated that in 20 hours, I'd gain around 5 mil in XP, so that's something at lvl98
Holy Freeze Merc. At least for the practice runs, I'll be keeping him. I guess vs your typical caster type runners like Sorc and Javazon, Hammerdin is a lot more hands on, where you spend a lot of time near cows or in the middle of the pack to make sure you hit with your hammers. At least for the few runs I did, Holy Freeze seemed very helpful when there's 10 cows swinging their poleaxes at you, so it's best that they don't do that super fast. Even with Max Block and high-ish defense, Hammerdin gets hit surprisingly often there, so limiting the attack speed of cows will help. Not sure how much Defiance Merc would help in terms of not getting hit that often, but I have a feeling that Holy Freeze works better since you have more time to teleport out of the pack if things get too hot. Also slows your runs down, but I am at a point where I just can't die anymore if I want to make 99.
edit.
Might be a silly question, but everyone's running /p7 right? Or is it similar to Travincal where some lower player setting will be better for efficiency?