i think crescent moon, which uses quite common runes, decreases enemy light res by about 30%. An act 2 merc would be able to use it in a polearm. Holy freeze is normally the best, you're traps will get better hits when enemies are slowed.
Infinity is too hard to get by even in SC. Surely, you can pay US$$$ to buy a premade Inifity wpn, but that's not what most ppl do. I've played SCL for a while now and did a dozen of full rushes myself for hellforge drops. But even my rushing sorc can only use a bonehew (it's enough for up to act4 in Hell in a not too big game) on her merc, and she found the wpn herself. It'll probably take me 20 full Hellforge rushes to get enough runes to trade for 2 BERs and either find or trade for an eth elite polearm wpn to make the runeword on. do you know how long this may take me to do? I have no problem getting a couple chars from act1 to act5 (or act4 in Hell) very quickly, but how to get lvl 1 babies complete norm/NM baal quests? Also surely, I could do MF for the same purpose, but it is very unlikely that i would get enough stuff for trading for an Infinity given the same amount of time for doing 20 rushes, whereas 20 rushes can more or less guarantee that.
the cold dmg of the cold merc can be ignored in Hell act4/5 on ladder. I know this b/c I had a sorc in the past who picked up this merc from Norm. He can freeze most monsters in Hell, and can survive really well as long as your player char is able to tank for him (in the case of a trapsin, that's the job for your SM). compared to any act2 merc, he is a poor match for a trapsin.
For an act2 tanking merc, stone made on a very high defense (7xx or 800+) armor is almost always the best choice. And that's a very cheap runeword compared to CoH or close.