Blaster rifles do an average of 24 point of damage per hit before taking resistances into account, which is on a par with the artifact sword Hercules. The advantage of blasters is that they have a recovery time getting down towards zero. However, this advantage only applies to real time combat, where the hostiles can move around. Auto-targeting defaults to the closest opponent, and an opponent that is getting hammered usually retreats, so that the target switches to a less severely damaged monster. The overall effect is that you have to damage the whole roomful before significant numbers of monsters start to drop. The Terminators don't blast every time, and the blast has to get through your defences, but if you don't cut down their numbers quickly, the odds of losing your character rise steadily as combat duration increases.
You can cut down those odds substantially by moving to real-time combat so that you can select individual Terminators and take them out one by one. OK, this is with a full party, not soloists, but I routinely clear the big room room using that technique with fully developed Dark Magic Shrapmetal casters, without losing any characters. I take out the Terminators first, and the rest die very quickly, when switch to real-time combat for the remainder.