ok firstly a warning 

if you want to only level up ONE guardian and use the solo skirmish function do NOT tick the automatically recharge energy option or the game will continue to recharge AFTER the guardian maxes out and burn through your energy recharges 

what you need to do to get round this is to use the group skirmish function 

add in the guardian you want to max out 

tick the automatically recharge energy box 

then click start skirmish 

this will stop once the guardian is at max without burning through recharges etc