stats.wesnoth.org - cheaters, be gone!

[jelle]

if you lose one of your best units, what do you do? continue whit your defastated army or reload the previous turn?

[SumnerH]

if you lose one of your best units, what do you do? continue whit your defastated army or reload the previous turn?

Continue, usually. If it's a really bad loss I might restart the scenario. Restarting turns in-scenario feels like cheating to me, so I don't do it.

[Feldegast]

if you lose one of your best units,

...then your strategy and/or tactic was not good enough. You should have protected your unit better. Move it on better ground, out of range, or shield it with cheap and inexperienced units. Always be prepared for bad luck and a lvl3 unit not hitting an enemy in one turn at all. In Battle for Wesnoth you need to learn how to make kamikaze moves only with units which you don't care about if they get killed. Sometimes it may be necessary to only recall very few "best units" and go into dangerous fights with new recruits, with the "best units" as supporters or to finish off wounded enemies.

what do you do? continue whit your defastated army or reload the previous turn?

Continue, of course. And see whether the lost unit was worth it. It may be that I can level up enough other units, so losing one may not be important.

Once you know a scenario, you can prepare better for it even earlier in the campaign by levelling up appropriate units.