You'll face a ton of Tickers once you reach Landown in the first Act. You need to melee them with a weapon other than the Lancer right as or just after they activate their flames. It seems to carry over if you die, so even if they kill you, you'll restart the area and you can continue. This can easily be unlocked before you complete the tunnel.
Also, they only attack by themselves at first, so you can move out from cover (watch your back) to melee them, since your allies have a bad habit of killing them when they're right next to you and dealing you damage.