[Keeper of Dol Guldur]

Silmarillion and LOTR class Balrogs are large, deadly, and have mid-ranged weapons (whips) and short ranged weapons (blades).

So the ideal way to kill them is to be in the mountains, on some high cliff that you can throw them off.

Luring them to mountains doesn't seem to be hard, they seem to be drawn to the high peaks. Just lead refugees on a mountain pass, and a Balrog will come. Then, all it takes is one being who has seen Valinor, using some leverage to knock the brutes over the precipice.

Make sure there isn't a huge lake below ... because the Balrogs will live.

But, if it's a fountain ... they'll die. Apparently.

Ecthelion drowned Gothmog in a fountain ... of course, Gondolin was in a mountainous region, and there may have been a cliff involved, it may have had sheer, steep walls that Gothmog couldn't just slime his way out of. That explains why Ecthelion couldn't get out either ... that and swimming in a suit of armor.

Perhaps the fountain of Gondolin was more like a really big well or reservoir.

Anyway, from a tactical point of view ... all it takes is a sword to parry blows and an indeterminable amount of strength to fight with the Balrog until a suitable cliff can be found.